BACKGROUND OF THE INVENTION

The invention relates to a method for calibrating a UWS quality sensor for a urea/water solution (UWS) which can be arranged in a UWS tank of a motor vehicle. Moreover, the present invention relates to a computer program which carries out all the steps of the method when it runs on a computing unit, and to a machine-readable storage medium which stores said computer program. Finally, the invention relates to an electronic control unit which is set up to carry out the method according to the invention.

SCR catalytic converter systems (SCR=Selective Catalytic Reduction) for nitrogen oxide reduction by means of metering in the urea/water solution (UWS) are known in the field of automotive technology. Future legislation in respect of emissions control requires monitoring of the quality of the UWS which is used by means of a UWS quality sensor, in order, in particular, to detect possible incorrect filling by way of the vehicle driver. The legal requirements for the accuracy of said quality monitoring correspond to a measuring accuracy of the sensor of ±1% mass fraction of urea in the UWS.

A known approach for said quality monitoring comprises ultrasonic sensors, by means of which the sound propagation time over a predefined distance is detected, and the urea content of the UWS is derived from the detected propagation time. This can be implemented in such a way that the ultrasonic sensor or transmitter and a sound reflector which is set up for this purpose are arranged in the medium at a defined spacing from one another, with the result that the different sound propagation times can be assigned to a medium density and, as a consequence of this, to a medium concentration. Said sensors are usually calibrated with a suitable UWS during manufacture. Owing to the high requirements for the measuring accuracy, a calibration is required in the case of the available sensors, which calibration influences the conversion rule and/or a characteristic diagram in such a way that the UWS concentration value is derived correctly from the measured value.

It is to be noted that said required measuring accuracy cannot be achieved with other less expensive sensors in the field of automotive technology under the given boundary conditions. It is to be taken into consideration in the case of a calibration that systematic influencing of the sensor accuracy by way of the geometric shape of the reservoir occurs as a result of the installation of the sensor, in particular as a result of thermal welding into the reservoir for the UWS. Therefore, different characteristics of the correction measures are to be provided in a manner which is dependent on the shape of the reservoir and/or the installed situation of the sensor or the sensor unit comprising a transmitter and a receiver.

Furthermore, it is to be emphasized that incorrect filling with UWS can lead via a corresponding sensor alarm not only to a warning or switch off of the SCR catalytic converter system, but rather to the automatic shutdown of the motor vehicle. A false alarm which has the same consequences should therefore be avoided in all circumstances.

SUMMARY OF THE INVENTION

The invention is based on the concept of carrying out a calibration of a UWS quality sensor which can be arranged or is arranged in a motor vehicle in an installed situation of the UWS quality sensor in the UWS tank. The UWS quality sensor can be integrated, in particular, into a supply module which is arranged in the UWS tank of the motor vehicle. In the installed situation, the UWS quality sensor or the supply module which receives it is fastened in the tank in the respective installed position. A calibration before the installation of the quality sensor in the tank is as a rule merely insufficiently feasible, since installation-induced length changes can occur as a result of the installation of the supply module or the quality sensor in the tank, which length changes influence the sensor behavior. In accordance with the invention, influences of the sensor accuracy which are brought about by way of the shape and embodiment of the UWS supply tank are determined, and are stored in a computing unit or electronic control unit in such a way that at least one correction measure which is adapted to this is retained for a respective UWS supply tank or a UWS supply tank shape. The correction measure for the UWS quality sensor can take place, in particular, in a manner which is dependent on the temperature of the UWS and/or the temperature of the tank as long as said temperatures exert an influence on the shape and/or the elastic properties of the UWS supply tank and, as a consequence, an influence on the UWS quality sensor which is connected to the supply tank, within the scope of operating limits. In particular, systematic influences on the UWS quality sensor can be corrected by way of said measure.

A concluding calibration of the quality sensor after the final installed position in the tank is reached takes place directly upon the initial start up of the system. The quality monitoring of the urea/water solution (UWS) in the tank can be ensured in the long term by means of said measures.

In one preferred refinement of the method, the calibration of the UWS quality sensor which is arranged in a motor vehicle is carried out in the case of an initial start up of the motor vehicle and/or after ensuring there has been no previous calibration of the UWS quality sensor. An initial start up comprises not only the first start of the vehicle, but rather can also comprise situations, in which a previously retrieved running performance of the vehicle lies below a threshold value, in particular below approximately 100 km.

A calibration in the case of the initial start up and in the case of a final installed situation of the quality sensor has the advantages that the system is capable of measuring here without additional complexity and no further production steps have to be carried out following the calibration, which further production steps might impair the calibration again. In addition, a change of the measured properties which is induced by way of the mounting of the quality sensor can be taken into consideration during the calibration. An additional calibration step which is associated with considerable costs is no longer necessary as a result. A step of this type might preferably be performed again when the quality sensor is replaced, for example within the context of a service measure.

In one preferred refinement of the method, the UWS quality sensor is detected, at least one feature of the UWS quality sensor being determined and being matched with a stored feature, a calibration of the UWS quality sensor being carried out only when the determined feature and the stored feature correspond or indicate the same UWS quality sensor. The feature can be the serial number of the quality sensor or another uniquely identifiable feature. Should it be detected as a result of the matching of the features that the quality sensor which is connected to the control unit is not that quality sensor, for which the calibration data are stored, a corresponding warning message can be output and/or the driver of the vehicle can be prompted to visit a repair shop immediately, in order to allow a corresponding recalibration of the quality sensor to be performed. Otherwise, it can also be determined within the context of sensor scanning of this type, in particular within the context of a self-test, whether there is a malfunction of the quality sensor and/or whether the latter is in a state, in which measured signals can be received and a calibration can be performed.

Expediently, after a start up of the vehicle, preferably after each start up of the vehicle, a check is carried out as to whether there is an initial start up of the motor vehicle. In the case of the detection of an initial start up of the motor vehicle and/or after ensuring that there has not been a preceding calibration of the UWS quality sensor, a measured value of the UWS quality sensor is detected. Subsequently, an adaptation of a conversion rule for said measured value into a UWS concentration takes place. Said adapted conversion rule is then stored permanently in an electronic control unit of the motor vehicle. As a result, a use of said conversion rule is possible even in the case of subsequent start ups of the motor vehicle.

It is also advantageous to fill a UWS tank of the motor vehicle with a urea/water solution of a predefined concentration before the initial start up of the motor vehicle, which concentration is taken into consideration during the adaptation of the conversion rule, since the properties and quality of the solution which is used are ensured by way of quality assurance measures with respect to the UWS supplier, as a result of which the conversion rule can be adapted in an improved manner and, as a result, the calibration of the sensor can be refined.

As an alternative, it is preferred to fill the UWS tank with a UWS before the initial start up of the vehicle, the concentration of which UWS is measured, and to take said measured concentration into consideration during the adaptation of the conversion rule. In this way, the concentration of the urea/water solution is determined very precisely and independently of manufacturing fluctuations, as a result of which an even more precise calibration of the UWS quality sensor is achieved.

Further preferably, for a calibration, a time period is waited after filling of the UWS tank with a urea/water solution and before a calibration, which time period is required to ensure a discharge of gas bubbles from the urea/water solution and/or to ensure thorough mixing of the urea/water solution and/or to ensure mechanical calming of the urea/water solution and/or to ensure a temperature equalization of the urea/water solution to the tank and its installed parts. Said step is preferably carried out after a determination of the identity of the quality sensor and before the actual calibration.

The above-described measures which can be used in an alternative or cumulative manner serve to establish an equilibrium state of the system with the aim of performing a measurement which is as reproducible as possible and, based on this, of performing a particularly lasting calibration of the quality sensor.

The adaptation of the conversion rule takes place, in particular, only when the measured value which is detected by the UWS quality sensor lies in a predefined value range. If the determined UWS concentration lies outside said value range, the calibration of the sensor is not permitted. In a case of this type, checking of the sensor and the UWS which is used for their specific properties can be considered, in order to avoid possible error chains during later operation.

Here, the value range is preferably selected in such a way that it corresponds to a concentration of the UWS in the range from 25% by weight to 40% by weight, preferably 35% by weight. As a result, possible incorrect calibrations as a consequence of a faulty quality sensor or pronounced quality fluctuations of the UWS can be prevented. If the value range is departed from, the possible fault sources can be checked, and the fault causes can be determined and pursued.

The computer program according to the invention makes it possible to implement the method according to the invention in an electronic control unit. To this end, it performs each step of the method according to the invention, in particular if it runs on a computer unit or electronic control unit. The machine-readable storage medium according to the invention stores the computer program according to the invention. By way of the computer program according to the invention being uploaded to an electronic control unit, the electronic control unit according to the invention is obtained which is set up to calibrate a UWS quality sensor by means of the method according to the invention.

The invention can be used, in particular, in conjunction with gasoline engines or diesel engines of motor vehicles which are equipped with an SCR exhaust gas aftertreatment system which has an abovementioned quality sensor for monitoring the UWS concentration. It goes without saying, however, that the method according to the invention can also be used in the case of corresponding internal combustion engines outside of automotive technology, for example in the field of marine technology or in the field of chemical engineering, with the advantages which are described herein.

Further advantages and refinements of the invention result from the description and the appended drawings.

It goes without saying that the features which are mentioned in the preceding text and are still to be described in the following text can be used not only in the respectively specified combination, but also in different combinations or on their own, without departing from the scope of the present invention.

