EP2619432B1 - Method and device for monitoring the function of an exhaust sensor - Google Patents

Description

State of the art

The invention relates to a method for monitoring the function of an exhaust gas sensor in the exhaust passage of an internal combustion engine.

The invention further relates to a corresponding device for monitoring an exhaust gas sensor in an exhaust passage of an internal combustion engine with a control unit of the internal combustion engine and the exhaust gas sensor associated control unit for controlling the internal combustion engine and for evaluating the output signals of the exhaust gas sensor.

Exhaust gas sensors are used today in various designs for monitoring the emissions of internal combustion engines. To ensure this function, the exhaust gas sensors must be checked for their functionality at regular intervals, for example as part of an on-board diagnostic (OBD). For the implementation of some of the necessary diagnostic functions certain operating conditions of the internal combustion engine must be present. Thus, for example, the plausibility of a measured oxygen concentration by a broadband lambda probe is preferably carried out in a coasting operation of the internal combustion engine in which no fuel is supplied to the internal combustion engine, since in an error case, the deviation of the sensor signal from an expected value is greatest.

From the EP0909888A1 a method is known in which a monitoring of the functionality and the aging of a linear oxygen probe is provided.

As part of new operating strategies for internal combustion engines and new technologies, the necessary for monitoring the function of the exhaust gas sensors Operating points of the internal combustion engine no longer approached in sufficient frequency. Thus, the internal combustion engine is switched off when operated according to the start-stop method motor vehicles in the state, so that the operating state idle is no longer or only very rarely present. In new technologies, such as the hybrid drive, the overrun operation is largely prevented.

In addition to the monitoring of the exhaust gas sensors are provided at certain intervals Lemfunktionen for the various exhaust gas sensors, such as a Schubadaption in broadband lambda probes. For this purpose, the necessary operating state, in the example mentioned the overrun, is also requested separately in hybrid vehicles or in start-stop systems. At the same time as these learning functions, the necessary diagnostic functions for the exhaust gas sensors can also be carried out. However, the performance of the diagnostic functions is limited to the duration of the learning function and the frequency with which it is performed.

It is an object of the invention to provide a method by which the function of exhaust gas sensors can be monitored, even if the necessary operating points of the internal combustion engine are rarely approached.

It is a further object of the invention to provide a device suitable for carrying out the method.

Disclosure of the invention

The object of the invention relating to the method is achieved in that in a first operating point of the internal combustion engine, a first functional check of the exhaust gas sensor determines the output signal of the exhaust gas sensor or a characteristic quantity derived therefrom and at least a second operating point of the internal combustion engine with an intact exhaust gas sensor and stored as a learning value and that the monitoring of the function of the exhaust gas sensor during a later operation of the internal combustion engine in the second operating point by comparing the output signal of the exhaust gas sensor or the characteristic derived therefrom with the learning value.

The first functional check is carried out in a suitable for monitoring the function of the exhaust gas sensor operating point of the internal combustion engine, which, however, is rarely approached. Here it can be reliably detected whether the exhaust gas sensor is in order. Subsequently, when the exhaust gas sensor is functioning, a second, more frequently approached operating point of the internal combustion engine is approached selectively or during normal operation of the internal combustion engine, and the learned value is detected at this operating point. The learning value is the output signal of the exhaust gas sensor in the second operating point or a characteristic derived therefrom. During later operation of the internal combustion engine, the monitoring of the function of the exhaust gas sensor can now be carried out during the frequently existing operating phases at the second operating point and thus at a sufficient frequency. For this purpose, the then existing output signal of the exhaust gas sensor or the characteristic derived therefrom is compared with the learned value. The learning values can be recorded for a second operating point or for any number of further operating points, so that a sufficiently frequent check of the function of the exhaust gas sensor can be ensured.

It is advantageous in the comparison of the current actual value of the output signal or the characteristic derived therefrom in the second operating point with the previously determined learning value that only changes with respect to the initial state must be detected and monitored. The overall tolerance of the system need not be taken into account, which enables monitoring of the function of the exhaust gas sensor outside the more suitable first operating point.

A simple comparison of the actual value of the output signal of the exhaust gas sensor or the characteristic derived therefrom with the learned value is made possible by assigning an upper and a lower threshold value to the learning value and by concluding a faulty exhaust gas sensor if the output signal of the exhaust gas sensor or the derivative thereof Parameter during monitoring exceeds the upper threshold or falls below the lower threshold. When specifying the threshold values, measurement inaccuracies can be added the determination of the output signal of the exhaust gas sensor and in the determination of the operating point and permissible changes in the output signal of the exhaust gas sensor, for example, by a permissible aging of the exhaust gas sensor, are taken into account.

A misdiagnosis of a defective exhaust gas sensor due to a single faulty measurement can be avoided by inferring a faulty exhaust gas sensor if a faulty exhaust gas sensor is repeatedly detected in successive monitoring phases.

To be able to reliably detect malfunctions of the exhaust gas sensor, it can be provided that an operating state of the internal combustion engine is selected as the second operating point, which is frequently started up and / or is approached over a sufficiently long period for monitoring the exhaust gas sensor and / or in which Exhaust gas sensor shows large deviations in a recognizable malfunction.

The second operating point can be unambiguously specified by the fact that the second operating point of the internal combustion engine by a speed or an injection quantity or an air mass or an exhaust gas recirculation state considered individually or in combination of the sizes.

According to a preferred embodiment of the invention, it may be provided that in the first operating point, the internal combustion engine is operated in a coasting operation and / or that in the second operating point, the internal combustion engine is operated under partial load. The overrun mode allows the absolute verification of various exhaust gas sensors, for example broadband lambda sensors, since no fuel is supplied to the internal combustion engine, the operating point is unambiguously described and a sufficiently accurate measurement of the sensor signal can be carried out for comparison with an unambiguously to be determined default value. Since the internal combustion engine is operated predominantly under partial load, a sufficiently frequent check of the function of the exhaust gas sensor can take place here in the second operating point. The tolerances for an absolute check of the exhaust gas sensor under partial load are too large; the invention However, relative evaluation by comparison with the previously determined learning value enables reliable detection of malfunctioning of the exhaust gas sensor even under partial load.

Depending on the operating strategy for the internal combustion engine and new technologies, such as the operation of the internal combustion engine after a start-stop method or use in a hybrid drive, certain operating points are approached only for the implementation of learning function for the various sensors used. During normal operation of the internal combustion engine, these operating points are no longer present. In order nevertheless to enable a first functional monitoring of the exhaust gas sensor, it may be provided that the first functional control of the exhaust gas sensor takes place during an operating phase of the internal combustion engine which is requested for carrying out a learning function for the exhaust gas sensor or for a further sensor.

The object of the invention relating to the device is achieved in that in the control unit a first program sequence which activates a first operating point of the internal combustion engine and during the first operating point carries out a first functional check of the exhaust gas sensor and drives at least one second operating point of the internal combustion engine and the exhaust gas sensor is intact Output signal of the exhaust gas sensor or a characteristic derived therefrom and stored as a learning value in the control unit is provided and that in the control unit, a second program sequence, which monitoring the function of the exhaust gas sensor during a later operation of the internal combustion engine in the second operating point by comparing the output signal of the exhaust gas sensor or the characteristic derived therefrom with the learning value is provided.

The first program sequence initially allows the monitoring of the function of the exhaust gas sensor according to known methods, so that it can be safely assumed for the subsequently provided determination of the learning values of a faultless exhaust gas sensor. For this purpose, a rarely approached first operating point, which allows a clear assessment of the functionality of the exhaust gas sensor, approached.

The learning value is recorded by determining the output signal of the exhaust gas sensor or a characteristic quantity derived therefrom in a second, more frequently approached operating point of the internal combustion engine. Since the learning value is determined for the current system, tolerances that preclude prediction of the learning value without direct measurement or the transmission of the learning value from one system to another can be neglected. With the second program sequence, therefore, the operability of the exhaust gas sensor in the second, frequently approached operating point of the internal combustion engine by comparing the current output signal or a characteristic derived therefrom with the learned value can be detected. The advantage here is that the functions can be implemented inexpensively by a pure software extension of the control unit using existing processor and memory units.

The method and the device can preferably be used for monitoring a lambda probe.

The method and the device can furthermore preferably be used for monitoring an exhaust gas sensor in the exhaust gas duct of an internal combustion engine operated in start-stop mode or an internal combustion engine used in a hybrid vehicle.

• Figur 1 ein erstes Ablaufdiagramm eines ersten Programmablaufs zur Bestimmung von Lemwerten,
• Figur 2 ein zweites Ablaufdiagramm eines zweiten Programmablaufs zur Überwachung der Funktion eines Abgassensors.

The invention will be explained in more detail below with reference to an embodiment shown in FIGS. Show it:

• FIG. 1 a first flow chart of a first program sequence for determining lem values,
• FIG. 2 a second flowchart of a second program sequence for monitoring the function of an exhaust gas sensor.

FIG. 1 shows a first flowchart of a first program sequence for determining learning values for monitoring the function of an exhaust gas sensor designed as a broadband lambda probe. The first program sequence is not in one deposited and associated with an internal combustion engine control unit deposited, wherein the internal combustion engine is a part of a hybrid drive.

In a first functional block 10, the internal combustion engine is operated in a pushing operation. The overrun operation is not provided in the regular operation of the internal combustion engine and is requested separately for carrying out a thrust adaptation of the broadband lambda probe. In addition to the thrust adaptation, in a second functional block 11 a first functional check of the broadband lambda probe takes place during the overrun operation suitable for the functional control of broadband lambda probes. In a first query 12 it is decided on the basis of the first function check whether the broadband lambda probe is in order. If this is not the case, the sequence branches off to a third function block 13 and a corresponding error message is issued. In the case of an intact broadband lambda probe, the first query 12 is followed by a fourth function block 14. In the fourth functional block 14, the internal combustion engine is operated under partial load at a second operating point. The second operating point can be approached targeted or detected during normal operation of the internal combustion engine. It can be described by the number of revolutions, the injection quantity, the air mass and the exhaust gas recirculation state. If the second operating point is present, the output signal of the broadband lambda probe is determined in a fifth functional block 15 and the lambda value determined therefrom is stored in a sixth function block 16 as a learning value for the second operating point.

FIG. 2 shows a second flowchart of a second program sequence for monitoring the function of the designed as a broadband lambda probe exhaust gas sensor in the FIG. 1 described technical environment. This second program sequence is also stored in the control unit. The second program sequence is activated, if by the in FIG. 1 described first program run the learning values for the second operating point are determined.

In a seventh functional block 20, the internal combustion engine is operated regularly. It is checked in a second query 21, whether the second operating point exists. If this is not the case, the further regular operation of the internal combustion engine takes place.

If it is determined in the second query 21 that the second operating point is present, the determination of the output signal of the broadband lambda probe and the conversion of the output signal into a lambda value takes place in an eighth function block 22. In a ninth function block 23, the thus determined actual value of the lambda value is compared with the learning value determined in the first program sequence for the second operating point. It is checked in a third query 24, whether the actual value of the lambda value is within a predetermined tolerance range to the learning value. If this is the case, it is assumed that an intact broadband lambda probe and the sequence jumps back before the seventh function block.

If the actual value of the lambda value is outside the tolerance range by the learning value, a defective broadband lambda probe is assumed. In order to increase the reliability of statements and to avoid faulty fault messages, a counter is first incremented by one increment in a tenth function block 25. In a fourth query 26 is queried whether the counter has reached a predetermined value N. If this is not the case, the process returns to the seventh function block 20. However, if the counter has reached the predetermined value N, a deviation of the actual value of the lambda value from the learned value outside the permissible tolerance has repeatedly been determined Broadband lambda probe is assumed. In an eleventh function block 27 then the diagnosis broadband lambda probe is defective with a corresponding error message.

The process is in the FIGS. 1 and 2 shown by way of example for the monitoring of the function of a broadband lambda probe, but can be applied mutatis mutandis to other exhaust gas sensors whose function monitoring preferably takes place in rarely approached operating points of the internal combustion engine. The monitoring of the function of the respective exhaust gas sensor can be carried out in one or more operating points, in which case the learning values must then be determined in the first program sequence for the various operating points.

In the case of the broadband lambda probe, for example, a load drop at the balancing line can be monitored. Such a load drop leads to a multiplicative Error on the oxygen concentration signal of the broadband lambda probe. The relative error is independent of the oxygen concentration to be measured. The absolute deviation from the expected broadband lambda probe output is greatest in overrun. Therefore, this error is monitored by a signal range monitoring of the output signal in overrun operation of the internal combustion engine or by a plausibility of the output signal against a calculated signal in overrun according to known methods.

For hybrid drives, it is envisaged that the overrun operation will only be taken on request by a learning function for the broadband lambda probe. This means that the frequency of diagnosis is significantly reduced. However, a load drop on the balance line has a large effect on the output of the broadband lambda probe. To monitor this error, it is checked in a phase in which the overrun operation is taken, whether the broadband lambda probe is in order. If it can safely be assumed that the broadband lambda probe is operating correctly, then learning values are recorded at partial load of the internal combustion engine for the oxygen concentration at one or more operating points. In this case, the output value of the broadband lambda probe or the lambda value formed therefrom is used as the learned value. Thus, a vehicle and probe specific learning value is determined for a given operating point. In the further course of the driving cycle, when no more overrun operation is taken, the compliance of this learning value is monitored by the monitoring function in the second program sequence. If the actual value of the output signal or the lambda value formed therefrom deviates from the learning value, it is possible to deduce a defective broadband lambda probe.

Claims (10)

1. Method for monitoring the function of an exhaust gas sensor in the exhaust duct of an internal combustion engine, characterized in that a first functional check of the exhaust gas sensor takes place at a first operating point of the internal combustion engine, in that, if the exhaust gas sensor is intact, the output signal of the exhaust gas sensor or a characteristic variable derived therefrom is determined at at least one second operating point of the internal combustion engine and is stored as a learning value and in that the monitoring of the function of the exhaust gas sensor during later operation of the internal combustion engine at the second operating point is performed by comparing the output signal of the exhaust gas sensor or the characteristic variable derived therefrom with the learning value.
2. Method according to Claim 1, characterized in that the learning value is assigned an upper and a lower threshold value and in that a faulty exhaust gas sensor is inferred if, during the monitoring, the output signal of the exhaust gas sensor or the characteristic variable derived therefrom exceeds the upper threshold value or drops below the lower threshold value.
3. Method according to Claim 1 or 2, characterized in that a faulty exhaust gas sensor is inferred if a faulty exhaust gas sensor is detected repeatedly in successive monitoring phases.
4. Method according to one of Claims 1 to 3, characterized in that an operating state of the internal combustion engine which is activated frequently and/or which is activated for a sufficiently long period of time to monitor the exhaust gas sensor and/or during which the exhaust gas sensor indicates large deviations when there is a detectable malfunction is selected as the second operating point.
5. Method according to one of Claims 1 to 4, characterized in that the second operating point of the internal combustion engine is defined by an engine speed or an injection quantity or an air mass or an exhaust-gas recirculation state, in each case considered in isolation or in a combination of the variables.
6. Method according to one of Claims 1 to 5, characterized in that the internal combustion engine is operated in trailing-throttle mode at the first operating point and/or the internal combustion engine is operated under partial load at the second operating point.
7. Method according to one of Claims 1 to 6, characterized in that the first functional check of the exhaust gas sensor takes place during an operating phase of the internal combustion engine that is requested to implement a learning function for the exhaust gas sensor or for a further sensor.
8. Device for monitoring an exhaust gas sensor in an exhaust duct of an internal combustion engine with a control unit assigned to the internal combustion engine and the exhaust gas sensor for controlling the internal combustion engine and for analysing the output signals of the exhaust gas sensor, characterized in that in the control unit there is provided a first program sequence, which activates a first operating point of the internal combustion engine and, during the first operating point, implements a first functional check of the exhaust gas sensor and, if the exhaust gas sensor is intact, activates at least one second operating point of the internal combustion engine and detects the output signal of the exhaust gas sensor or a characteristic variable derived therefrom and stores it in the control unit as a learning value, and in that in the control unit there is also provided a second program sequence, which implements the monitoring of the function of the exhaust gas sensor during later operation of the internal combustion engine at the second operating point by comparing the output signal of the exhaust gas sensor or the characteristic variable derived therefrom with the learning value.
9. Method according to one of Claims 1 to 7 for monitoring a lambda probe.
10. Method according to one of Claims 1 to 7 for monitoring an exhaust gas sensor in the exhaust duct of an internal combustion engine operated in start-stop mode or an internal combustion engine used in a hybrid vehicle.

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