JPH0626330A - Method and device for discriminating state of catalyst - Google Patents

Abstract

PURPOSE: To accurately discriminate the quality of a catalyst by maintaining hyperoxia and oxygen deficiency to the catalyst at an average value until a composition of a gas mixture varies and increasing the same so as to estimate oxygen-storing ability of the catalyst. CONSTITUTION: In an internal combustion engine 3, fuel-metering means 1 is attached to a suction pipe 2, and exhaust sensors 4, 5 are arranged respectively before and behind a catalyst 6 in an exhaust pipe 7. Further, in addition to means 9 for comparing values detected by the exhaust sensors 4, 5 with a target value generated by means 10, a closed loop controller 11, multiplication means 12 and control means 13 are provided. In this case, a composite section 8 is provided. Furthermore, in the composite section 8, means 19 for comparing a threshold value of a detector 17 with a frequency of a detector 18, and means 20 for outputting a defect signal when the threshold is exceeded are disposed. Thus, defects of the catalyst 6 are discriminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for diagnosing the condition of a catalyst used for reducing emission of harmful substances (on-board diagnosis) during operation of an internal combustion engine.

[0002]

2. Description of the Related Art It is desirable to identify and replace catalysts with reduced conversion capacity in order to reduce the emission of harmful substances as much as possible. That is, there is a legislative demand for a system capable of diagnosing a decrease in conversion capability by means of on-board means (on-board diagnosis). To that end, in known systems (DE-P 40 24 210, US 46228
09), the signals from the first lambda sensor, which is arranged in front of the catalyst and is also used as a control sensor, and the second exhaust gas sensor, which is arranged behind the catalyst, are compared and used for diagnosis.

For this purpose, and also due to the two-position operation (on-off operation) characteristic of the air-fuel mixture closed-loop control method, the periodic oscillation of the lambda value which is inevitably generated at least in a steady operating state is used. The ideal value of this vibration corresponds to a control target value, for example lambda = 1, in the ideal case. Exhaust gas is in excess of oxygen in a half cycle of the vibration in which the air-fuel mixture becomes lean, while in the other half cycle, oxygen becomes deficient due to the rich mixture composition. On the basis of this oxygen storage capacity, the catalyst mitigates oxygen deficiency and oxygen excess within certain limits, thereby exerting an averaging effect on the oxygen content of the exhaust gas. In the known method, this effect is detected by comparing the amplitudes of exhaust gas sensors arranged in front of and behind the catalyst and is used for determining the conversion capacity.

A disadvantage of the above-mentioned method is that the signal amplitude is significantly dependent on the temperature of the individual sensors of the two sensors and other individual parameters such as chemical and thermal aging. Therefore, the determination of the conversion ability by this method has a corresponding uncertainty. As a result, it becomes impossible to identify catalysts that have failed to meet legal requirements, resulting in increased emission of harmful substances.

It is expected that this drawback will be magnified with increasing demands on the quality of the exhaust gas as well as on the monitoring of the system components important for the exhaust gas. In other words, if the limit value is further rounded down, the amount of harmful substances obtained from the vehicle under test conditions will be increased even when the signal of the rear sensor does not change significantly in normal closed-loop control operation. However, it is possible that the limit is exceeded.

[0006]

The object of the present invention is to provide a catalyst which does not have the above-mentioned uncertainty and which can provide reliable information on the quality of the catalyst even when the demand becomes more severe as mentioned above. It is to provide a method and apparatus for determining conversion ability.

[0007]

In order to solve this problem, the present invention provides a two-position lambda closed loop control device and a first exhaust gas sensor which is arranged in front of a catalyst and is used as a control sensor. Second located behind the catalyst
In a method for determining the oxygen storage capacity (conversion capacity) of a catalyst that reduces the emission of harmful substances in an internal combustion engine equipped with the exhaust gas sensor of the The mixture quantity) and the oxygen deficiency (rich mixture composition) input quantity continuously and until the change of the mixture composition is recognized in the signal of the exhaust gas sensor arranged behind the catalyst. It is increased symmetrically, that is, maintaining the average value, the oxygen storage capacity of the catalyst is estimated from the degree of symmetrical increase, and a catalyst defect signal is output when the oxygen storage capacity is no longer sufficient. did.

Further, according to the present invention, a lambda closed loop control device for continuous operation, a first exhaust gas sensor arranged in front of the catalyst and used as a control sensor, and a second exhaust gas sensor arranged behind the catalyst. Storage capacity (conversion capacity) of catalysts that reduce the emission of harmful substances in internal combustion engines equipped with
In this method, excess oxygen (lean air-fuel mixture composition) and oxygen deficiency (rich air-fuel mixture) are input alternately to the catalyst periodically, and these input amounts are placed behind the catalyst. Until a change in the mixture composition is observed in the signal of the exhaust gas sensor, which is continuously and symmetrically increased, that is, the average value Lambda = 1 is maintained, and the degree of symmetrical increase of the catalyst A configuration was also adopted in which the oxygen storage capacity was estimated and a catalyst defect signal was output when the oxygen storage capacity was no longer sufficient.

Further, in the device of the present invention, a lambda closed loop control device, a first exhaust gas sensor arranged in front of the catalyst and used as a control sensor, and a second exhaust gas sensor arranged behind the catalyst are provided. In an apparatus for determining the oxygen storage capacity (conversion capacity) of a catalyst that reduces the emission of harmful substances of an internal combustion engine equipped, the oxygen excess input amount and the oxygen deficient input amount continuously increase in order to determine the oxygen storage capability. Means for changing the fuel metering signal, further means for comparing the signal of the exhaust gas sensor in the rear with a predetermined threshold value, and means for detecting whether the threshold value is passed at a minimum frequency. And means for outputting a signal characterizing the state of the catalyst.

Further, the present invention includes a lambda closed loop control device, a first exhaust gas sensor which is arranged in front of the catalyst and is used as a control sensor, and a second exhaust gas sensor which is arranged behind the catalyst. In a system that produces a defective signal in relation to the conversion capacity of the catalyst of an internal combustion engine, the fuel metering signal oscillates around the mean value in the test phase,
Moreover, the values of the areas located above and below the average value formed by the straight line representing this average value and the time waveform of the fuel metering signal between the two points intersecting this straight line are the exhaust gas arranged behind the catalyst. It is modulated symmetrically with the increase of the test phase until it shows a change in the mixture composition in the sensor signal, i.e. maintaining the mean value and increasing, and the amount of increase made up to that point is the limit. The value is compared, and if this limit value is reached, a catalyst defect signal is output.

[0011]

Preferred embodiments of the invention are the subject of the dependent claims.

[0012]

An embodiment of the method according to the invention and the corresponding device is shown in the drawing and will be explained in more detail below.

FIG. 1 shows a fuel metering means indicated by reference numeral 1 provided in an intake pipe 2 of an internal combustion engine 3. In the exhaust gas pipe 7 of the internal combustion engine, exhaust gas sensors 4 and 5 are arranged in front of and behind the catalyst 6, respectively. Further, there are provided comparison means 9 for comparing the output signal of the exhaust gas sensor 4 with a control target value, target value generation means 10, closed loop controller 11, multiplication means 12 and control means 13 for generating a preset control value. . To this device already known from the prior art, a composite part 8 indicated by a broken line is added.

This composite section stores and outputs a control parameter or control value for adjusting the characteristics of the closed loop controller 11, a sequence control means 14, a filter 16 for the signal of the exhaust gas sensor 5, and a threshold. Value detector 17
A means 18 for detecting the frequency H exceeding the threshold value of the detector 17, and a means 19 for comparing the frequency H with the threshold value Hmin.
And means 20 for outputting or storing a defect signal when the threshold value Hmin is exceeded, and optionally a control factor FR
And means 20 for averaging.

A preset control value tp from the operating control means 13 of the internal combustion engine is multiplied in the multiplication means 12 by the control coefficient FR. The fuel metering signal ti thus obtained drives the fuel metering means 1, for example, the injection valve. The mixture of fuel and air formed in the intake pipe 2 burns in the internal combustion engine 3 and then flows as exhaust gas into the exhaust pipe 7, where the oxygen content of the exhaust gas is detected by the exhaust gas sensors 4 and 5, respectively.

The signal of the exhaust gas sensor 4 is compared with the target value from the target value generating means 10 in the comparing means 10. The comparison result is used as an input quantity of the PI closed loop controller 11, and this controller outputs the control coefficient FR as an output signal.
Is output. Often controlled to a lambda value of 1. This is because this value optimizes the conversion of various harmful substances in the catalyst 6. The oxygen content of the exhaust gas changed by the catalytic reaction is detected by the second exhaust gas sensor 5. It is shown that the exhaust gas sensor 5 is connected to the closed loop controller 11 to perform the known closed loop control by the exhaust gas sensor 5, which makes it possible to compensate the error of the front sensor 4 to some extent. .

The control circuit including the catalyst, the first exhaust gas sensor arranged in front of the catalyst as a control sensor, and the second exhaust gas sensor for detecting the oxygen content of the exhaust gas behind the catalyst has been described above. In the range, it is already known.

To carry out the method according to the invention, the elements described below, which consist of a complex 8 surrounded by a dotted line, are used, which are activated in a given operating state in order to carry out the invention. . When the closed loop control of the two-position operation (on / off operation) during normal operation of the internal combustion engine is used, the switch 2 is used.
The test phase is initiated by the sequence control means 14 via 2. After that, the means 15 for adjusting the control coefficient FR by open loop control acts on the control vibration, and, for example, the amplitude of the control vibration (FR vibration) is sequentially increased in a predetermined manner.

When the mixture composition is modulated by open loop control as a test function, it is preferable to average the control coefficient FR before starting the test and start the test with this averaged value. Means 21 are provided for this purpose. A signal filter 16 is arranged between the exhaust gas sensor 5 and the threshold value detector 17 to allow only a relatively rapid signal change of the exhaust gas sensor 5 and to block a slow change. This event is recorded by means 18 each time a rapid change occurs in the signal of the exhaust gas sensor 5 and the amplitude of the change increases and passes both thresholds TVI and TVII, whereby the frequency signal H of this event is recorded. Is formed. Rapid changes in the signal of the rear sensor 5 occur especially when a change (change) in the mixture composition is still visible behind the catalyst. The threshold detector 19 compares the value H with a predetermined threshold Hmin. When H exceeds the threshold value Hmin, that is, when the signal of the rear sensor 5 passes the threshold values TVI and TVII at a predetermined minimum frequency, the defective signal is output by the means 20 or the defective signal is used for later use. The signal is stored. At the same time, the test phase is terminated by the sequence control means 14 via the connection line of the threshold detector 19.

The method of the present invention will be described with reference to FIG. 2 which illustrates various signal waveforms with respect to time. Signals (FR, U) that inevitably occur due to the gas passage time and generate lambda vibrations in closed loop control operation of two-position operation, for example.
The time lag between SV, USH) is not shown.

FIG. 2 (a) shows the waveform of the control coefficient FR, FIG. 2 (b) shows the corresponding signal USV of the front exhaust gas sensor 4, and FIGS. 3 shows the corresponding signal waveform USH of the rear exhaust gas sensor 6 which occurs during various aging stages of the catalyst 6 during the implementation of the inventive method.

When the two-position closed loop control during normal operation is performed, the control coefficient FR oscillates around a value of 1. In this regard, the time interval (t = 0, t = 0,
t = t0). In the period when FR is greater than 1,
The mixture becomes rich and is therefore oxygen deficient. During this period, oxygen stored in the catalyst 6 in advance is destroyed. Therefore, the area shaded on the line of FR = 1 is the value of the oxygen deficient input amount to the catalyst 6, that is, the value of the oxygen amount which is generally lacking in the mixture for stoichiometric composition. Can be considered. Similarly, the area below the line of FR = 1 (dilute phase) represents the excess oxygen input amount. In the following, these input quantities are also called oxygen deficiency as well as oxygen excess.

Before the start of the test phase at time t0, in the illustrated embodiment, the closed-loop control is usually performed with a large amplitude according to the principle of two-position operation. In the oxygen-rich phase described above, the catalyst absorbs oxygen from the exhaust gas, and in the oxygen-deficient phase, oxygen is released again to the exhaust gas. Therefore, the catalyst has a compensating (buffering) effect on the oxygen content of the exhaust gas and averages the control oscillation during normal operation. In this case, (c) of FIG.
As shown in (d), the signal waveform of the sensor 5 behind the catalyst is not yet in the shape of the signal waveform of the front sensor.

Subsequently, at time t0, a test phase in which the integral slope (slope) is sequentially increased is started. As a result, in this embodiment, as is clear from FIG. 2A, the amplitude of the control vibration gradually increases. In that case, the increase takes place approximately symmetrically with respect to the FR = 1 line, ie the average value of FR is maintained.

Accordingly, the oxygen deficiency and the oxygen excess input amount, which are alternately generated, also increase symmetrically. As is apparent from FIG. 2 (b), the exhaust gas sensor 4 arranged in front of the catalyst does not show a significant reaction to the increase in amplitude. The signals USV respectively indicate changes from dark to light or vice versa. In that case, a low signal level characterizes the oxygen excess.

FIG. 2 shows the corresponding signal USH of the rear exhaust gas sensor 5 for two aging stages with different catalysts.
The characteristics are different in (c) and (d).

In the period shown by the time points t0 and t1c to t1d in the figure, the increasing oxygen excess amount and the oxygen excess amount can be compensated by the averaging action of the catalyst.

However, the averaging effect is limited by the final oxygen storage capacity of the catalyst.

When the increasing oxygen excess and oxygen excess exceed the limit of this averaging action, the oscillation of the control factor FR is also visible in the signal USH of the rear sensor. In FIGS. 2C and 2D, the thresholds TVI and TVII are passed at the times t1c to t1d.

In catalysts whose conversion capacity no longer corresponds to the requirements of the law, any increase in amplitude cannot be compensated by the averaging effect of the catalyst. Since the increase amount increases in synchronization with the increase in the test period, the period Δt = t
1-t0 can be used to determine oxygen storage capacity or conversion capacity. The lower the oxygen storage capacity of the catalyst, the earlier the time point indicated by t1c to t1d in FIG. Each of the periods tg shown is adapted to the applicable legislative requirements for the quality of the catalyst. Therefore, in the example of FIG. 2 (c), the catalyst is no longer sufficient for the demand, but in the example of FIG. 2 (d) it is still determined to be good.

Furthermore, by measuring the period Δt, a continuous value indicating the remaining conversion capacity of the catalyst can be obtained.

It should be noted that the increase in the oxygen deficiency input amount or the oxygen excess input amount is performed until the signal of the rear exhaust gas sensor jumps (alternates). In this way, according to the present invention, the jump characteristic of the exhaust gas sensor located at a certain degree rearward can be used for catalyst diagnosis. The advantages of the method according to the invention can be seen because the jumping characteristics of the output signal of the exhaust gas sensor show a relatively good stability against temperature changes and aging phenomena.

In reality, even with a good catalyst, the sensor signal individually changes due to a statistical fluctuation phenomenon.
Therefore, it is observed whether the comparison thresholds TVI, TVII have been passed with a predetermined minimum frequency H> Hmin within a predetermined time period, in order to enable reliable information on the catalyst state. Furthermore, it should be noted that the amplitude increase is relatively rapid due to drawing reasons. In practice, because of the resolution of the method, the increase is selected slightly so that the signal of the front exhaust gas sensor 4 passes the signal level more frequently than shown between the start and end of the test.

In FIG. 2A, the oxygen deficiency and excess amount are increased by, for example, increasing the integral slope.

However, the present invention is not limited to this particular signal waveform and includes all other signal waveforms that increase oxygen deficiency and excess. Another example of the control that acts on the mixture formation is shown in FIG. 3A, and in this example, the proportional change amounts (proportional components) P1, P2, ...
... are increased sequentially.

FIG. 3 (b) shows another example in which the delay time tIS gradually increases. During this delay time, the output signal of the closed loop controller 11 is the front sensor 4
Is held constant after the change of the output signal of (integration stop).

FIG. 3C shows an embodiment of the delay time tv which becomes effective after the change of the output signal of the front sensor 4 as well. In this example, the periods tv1, tv2, ...
During this period, the integration is continued with the same integration direction and integration slope, respectively. The start of the period tvi is indicated by the end of each dead time tt.

Besides the closed-loop control of the mixture formation, the method of the invention can also be implemented by modulating the fuel metering signal by open-loop control. As an example, FIG. 3D shows the characteristic of the sine-shaped coefficient FR in which the amplitude increases.

Using the example of modulating the mixture (composition) with open loop control as the test function, the test results can be erroneous if the actual lambda mean value is not chosen as the starting point for the increase. . Therefore, in order to eliminate this erroneous diagnosis, the value of the control coefficient FR or the signal of the front exhaust gas sensor is averaged over a few vibration cycles before the start of the increase, and the obtained value is the start value of the increase. It is preferable to use To that end, means 21 are provided.

When the closed loop control of continuous operation during normal operation is executed, one of the illustrated methods (two-position operation closed loop control, open loop control of air-fuel mixture) is executed as a test function.

FIG. 4 shows an example of a flow chart for implementing the method of the invention under computer control. For this purpose, first, in step S1, the sequence control 14 checks whether or not the preconditions necessary for conducting the test are satisfied. The premise is that the minimum operating time has elapsed since the last test, in order to prevent the test from being performed unnecessarily frequently, that both sensors are in operation, that the internal combustion engine and the catalyst are It is possible that the temperature is sufficient and that the load is almost constant and a small operating state, for example, an idling operating state exists. If one of these assumptions is not met, normal operation is maintained (step S10). In other cases, in step S2, if necessary, the control coefficient FR is averaged beforehand by the means 21, and then the oxygen deficiency and the oxygen excess are increased almost symmetrically.

After the start time t0 = t (start) is set in step S3 by a timer and a memory element (not shown),
In step S4, it is checked whether the frequency H determined by the means 18 exceeds the threshold value Hmin of the means 19. Period t
If so before g, the catalyst is estimated to be defective and a corresponding defect signal is output in step S9. On the other hand, if the frequency Hmin has not yet been exceeded, the process goes to step S5, where the period Δt = t1−t.
0, that is, the elapsed time after starting the test is obtained. If this time is shorter than the predetermined limit value tg, the process goes through the judgment step S6 and returns to step S4. On the other hand,
If Δt exceeds the value of tg without H reaching the limit value Hmin, the loop is exited. In this case, the increase is stopped in step S7, and step S8 is performed.
Press to return to normal closed loop control operation.

According to the above-described process flow, a "yes / no" answer can be obtained as to whether or not the conversion capacity of the catalyst still complies with the regulations of the law. However, according to the processing flow of FIG. 5 described below, it becomes possible to obtain a continuous value for the quality of the catalyst.

Following the flow of processing from FIG. 4 before the mark A, the test period Δt = t1−t0 is detected in step S11. The value H is the threshold value H in step S12.
It is checked whether it exceeds min. When Hmin is exceeded, the loop formed by both steps is released. "Yes / No" regarding the quality of the catalyst in the following step S13.
Is judged. If the period Δt is smaller than the limit value tg, the catalyst defect signal is output in step S15. On the other hand, when the period Δt becomes longer than tg, the signal A = A (Δt) is formed. This signal is shown in Figure 6.
As shown in the figure, continuous values for the quality of the catalyst are shown. The larger Δt is, the larger A is. Thus, A = 100% is characteristic of the fresh catalyst, while the value A = 0% means the catalyst is at the limit of aging. By thus obtaining a continuous value for the quality of the catalyst, it is possible to estimate the expected life of the catalyst. For example, if the inspection interval is fixed during maintenance service or check inspection, it can be deduced whether the catalyst is still functioning before the next inspection, possibly still without failure, or has to be replaced in advance.

[0045]

As described above, according to the present invention, it is possible to provide reliable information on the quality of the catalyst even when the legal requirements become more severe.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the device of the present invention combined with a known control target for controlling a fuel / air mixture of an internal combustion engine.

2 (a) to 2 (d) are waveform diagrams showing signal characteristics generated at various points in the device shown in FIG.

3 (a) to 3 (d) are waveform charts showing signal characteristics generated when the method of the present invention is carried out.

FIG. 4 is a flow chart diagram for implementing the method of the present invention under computer control.

FIG. 5 is a flow chart diagram showing another embodiment for carrying out the method of the present invention under computer control.

FIG. 6 is an explanatory diagram showing quality values of a catalyst.

[Explanation of symbols]

 1 Fuel Measuring Means 3 Internal Combustion Engine 4 Front Exhaust Gas Sensor 5 Rear Exhaust Gas Sensor 6 Catalyst 10 Target Value Generating Means 11 Closed Loop Controller 14 Sequence Control Means 16 Filters 18 Frequency Detecting Means 21 Average Value Forming Means

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Eberhard Schneibel 7241 Hemingen Hochstaetterstraße 1/5 (72) Inventor Erich Schneider 7125 Kirchheim Storchenweg 4

Claims (12)

[Claims] 1. A two-position lambda closed loop control device, a first exhaust gas sensor arranged in front of the catalyst and used as a control sensor, and a second exhaust gas sensor arranged behind the catalyst.
In the method for determining the oxygen storage capacity (conversion capacity) of a catalyst that reduces the emission of harmful substances in an internal combustion engine equipped with the exhaust gas sensor of The mixture quantity) and the oxygen deficiency (rich mixture composition) input quantity continuously and until the change of the mixture composition is recognized in the signal of the exhaust gas sensor arranged behind the catalyst. It is increased symmetrically, that is, maintaining the average value, and the oxygen storage capacity of the catalyst is estimated from the degree of symmetrical increase, and a catalyst defect signal is output when the oxygen storage capacity is no longer sufficient. A method for determining a catalyst state. 2. A lambda closed loop controller for continuous operation,
Oxygen storage capacity of a catalyst for reducing emission of harmful substances in an internal combustion engine equipped with a first exhaust gas sensor arranged in front of the catalyst and used as a control sensor and a second exhaust gas sensor arranged behind the catalyst In the method for determining (conversion capacity), excess oxygen (lean mixture composition) input and insufficient oxygen (rich mixture composition) input to the catalyst are periodically alternated, and these input amounts are Is increased continuously and symmetrically, ie maintaining the mean value Lambda = 1, until a change in the mixture composition becomes visible in the signal of the exhaust gas sensor arranged behind the A catalyst state determination method, wherein the oxygen storage capacity of the catalyst is estimated from the degree, and a catalyst defect signal is output when the oxygen storage capacity is no longer sufficient. 3. The change of air-fuel mixture is brought about by performing a test function, the test function then being to implement a closed loop control of two-position operation. Method. 4. When the catalyst is defective, the increase is carried out until a change in the air-fuel mixture composition can be recognized even in the signal of the exhaust gas sensor arranged behind the catalyst, and when it is not recognized, the catalyst is increased. The method according to any one of claims 1 to 3, characterized in that is estimated to be normal. 5. The method according to claim 1, wherein the increase of the input amount is performed by changing a control parameter during closed loop control operation. 6. Method according to claim 5, characterized in that the proportional component of the closed-loop control is increased. 7. A method according to claim 5, characterized in that the delay time which becomes effective following a sensor signal change is increased. 8. The method according to claim 7, characterized in that the value of the control factor FR or the value of the integral slope is held constant during the delay time. 9. The method according to claim 1, wherein the increasing of the input quantity is performed by modulating the fuel metering signal by open loop control. 10. The method according to claim 1, wherein the method is carried out only in certain operating conditions, for example when the internal combustion engine has a low load or is idling. . 11. A harmful effect of an internal combustion engine comprising a lambda closed loop control device, a first exhaust gas sensor arranged in front of the catalyst and used as a control sensor, and a second exhaust gas sensor arranged behind the catalyst. In a device that determines the oxygen storage capacity (conversion capacity) of a catalyst that reduces substance emission, in order to determine the oxygen storage capacity, a fuel metering signal is sent so that the oxygen excess input amount and oxygen deficient input amount continuously increase. Means for changing are provided, means for comparing the signal of the exhaust gas sensor at the rear with a predetermined threshold value, and means for detecting whether or not this threshold value is passed with a minimum frequency are further provided, and the catalyst is further provided. A catalyst state determination device, characterized in that means for outputting a signal characterizing the state is provided. 12. A catalyst for an internal combustion engine, comprising a lambda closed loop control device, a first exhaust gas sensor arranged in front of the catalyst and used as a control sensor, and a second exhaust gas sensor arranged behind the catalyst. In a system that produces a defective signal in relation to the conversion capacity of the fuel, the fuel metering signal oscillates around the mean value in the test phase, and the fuel between the straight line representing this mean value and the two points intersecting this straight line The values of the areas located above and below the average value formed by the time waveform of the metering signal are measured until the change in the mixture composition is recognized in the signal of the exhaust gas sensor located behind the catalyst. Is symmetrically increased with an increase in the mean value, that is, the mean value is maintained and increased, and the amount of increase made up to that time is compared with a limit value. System characterized in that the signal is output.