Abstract:
A method for the self-diagnosis of an exhaust gas probe ( 20 ). The exhaust gas probe ( 20 ) has at least one heating element ( 26 ). The method includes a modeled heat output PH is determined for the heating element ( 26 ) by means of a computer program comparing parameters of the gas stream with a measured heat output, and determining, when the parameters of the gas stream exceeds the measured heat output by more than a defined tolerance limit, that one of the exhaust gas probe ( 20 ) is defective, and an exhaust gas section ( 17 ) in which the exhaust gas probe ( 20 ) is installed is faulty.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for the self-diagnosis of an exhaust gas probe, the exhaust gas probe having at least one heating element. 
         [0002]    Furthermore, the invention relates to an apparatus for the self-diagnosis of a corresponding exhaust gas probe, the exhaust gas probe being connected to an engine controller or a sensor control unit, and the engine controller or the sensor control unit having devices for the diagnosis of the exhaust gas probe. 
         [0003]    Particle sensors are used nowadays, for example, for monitoring the soot emission of internal combustion engines and for on board diagnosis (OBD), for example functionally monitoring particle filters. Here, collecting, resistive particle sensors (particle mass sensors or PM sensors) are known which evaluate a change in the electric properties of an interdigital electrode structure on account of particle accretions. Two or more electrodes can be provided which preferably engage into one another in the manner of a comb. The electrodes are short circuited as a result of a rising number of particles accreting on the particle sensor, which results in an electric resistance which decreases as the particle accretion increases, a decreasing impedance or in a change in a characteristic variable which is linked to the resistance or the impedance, such as a voltage and/or a current. For evaluation, generally a threshold value, for example of a measuring current between the electrodes, is fixed and the time until the threshold value is reached is used as a measure of the accreted particle quantity. As an alternative, a signal change speed during the particle accretion can also be evaluated. If the particle sensor is fully loaded, the accreted particles are burned in a regeneration phase with the aid of a heating element which is integrated into the particle sensor. 
         [0004]    DE 101 33 384 A1 describes a resistive particle sensor of this type. The particle sensor is constructed from two comblike electrodes which engage into one another and are covered at least partially by a trapping sleeve. If particles from a gas stream accrete on the particle sensor, this leads to a change in the impedance of the particle sensor, which change can be evaluated and from which the amount of accreted particles and therefore the amount of particles entrained in the exhaust gas can be deduced. 
         [0005]    DE 10 2005 015103 A1 describes one special embodiment of a particle sensor of this type, in which embodiment the actual soot sensor is surrounded with a metallic housing for protection. Inter alia, one embodiment is described, in which the soot sensor is arranged in a protective tube which surrounds it or, as an alternative, a double protective tube which has one or more inlet openings for the gas stream. 
         [0006]    In order to regenerate the particle sensor after particle accretion has taken place, the sensor element has to be burned free with the aid of an integrated heating element. This has to be carried out at defined time intervals, in order to avoid corruptions of the particle concentration determination. 
         [0007]    For the self-diagnosis, there is provision, for example, in a further application of the applicant with the internal reference R.318399 for the particle sensor to have an additional flat test electrode, and for different test voltages to be applied between the measuring electrodes and the test electrode in a plurality of method steps, and for in each case a current or a capacitance to be measured and for correct functioning of the particle sensor to be deduced using the determined values. 
         [0008]    EP 1 925 926 A1 describes an apparatus and an evaluation method for testing the functionality and for carrying out a plausibility check of a sensor which is based on an electrode system, in particular of a particle sensor, the apparatus comprising at least one reference electrode system. Here, the evaluation method provides comparative measurements at the measuring electrodes and at the reference electrodes, correct functioning of the sensor being deduced using the determined values. 
         [0009]    Since the particle sensor is arranged downstream in the exhaust gas stream with regard to a particle filter if used for on board diagnosis, no more particles, in particular soot particles, which could supply a corresponding sensor signal, should be situated in the exhaust gas in the case of a fully functional particle filter at that point, at which the particle sensor is arranged. However, the fact that no signal is supplied by a sensor can also mean that the particle sensor is defective and therefore a possibly likewise defective particle filter is not detected as being defective. In addition, sooting of the protective tube can also lead to a faulty interpretation of the signals. 
       SUMMARY OF THE INVENTION 
       [0010]    It is therefore an object of the invention to provide a method which leads to improved self-diagnosis of an exhaust gas probe, in particular of a particle sensor. 
         [0011]    Furthermore, it is an object of the invention to provide a corresponding apparatus for carrying out the method. 
         [0012]    The object which relates to the apparatus is achieved by the fact that the engine controller or the sensor control unit has at least one calculating unit for the determination in a modeled manner of a parameter which denotes the correct operation of the exhaust gas probe, it being possible for this parameter to be determined from boundary conditions for the gas stream or from measured parameters for the gas stream, and it being possible for said parameter to be compared with a measured parameter by means of at least one evaluation unit which is integrated into the engine controller or into the sensor control unit, and it being possible for a defect of the exhaust gas probe to be derived from a deviation which exceeds a defined tolerance limit or it being possible for a faulty condition to be detected in an exhaust gas section, in which the exhaust gas probe is installed. Here, in one preferred design variant, the parameter which is defined in a modeled manner and is compared with the measured parameter with regard to the self-diagnosis is the heat output P H  of the heating element of the exhaust gas probe. 
         [0013]    Here, the method according to the invention provides for a modeled heat output P H  or a value which represents the modeled heat output P H  to be determined for the heating element by means of a computing program from boundary conditions for the gas stream or from measured parameters for the gas stream, and to be compared with a measured heat output or with a value which represents the measured heat output P H , and in the case of a deviation which exceeds a defined tolerance limit, for a defect of the exhaust gas probe to be deduced or a faulty condition in an exhaust gas section, in which the exhaust gas probe is installed, to be concluded. There can be provision here for the deviation to be evaluated with respect to defined limit values or for the absolute value to be evaluated in relation to a fixed threshold or a threshold value. 
         [0014]    One preferred method variant and one preferred apparatus variant provide here for a particle sensor for determining a particle content in the gas stream to be used as exhaust gas probe or the exhaust gas probe to be configured as a particle sensor, the particle sensor being heated by means of the heating element in a regeneration phase and a soot loading on the particle sensor being removed in the process. 
         [0015]    As a result of self-diagnosis of this type, it is possible, for example, to detect defective particle sensors, the electrodes of which have been damaged during the production or in use over the service life of the sensor, and to inform the driver/operator of the vehicle/system by way of visual and/or acoustic warnings if the sensor has failed totally. One preferred use of the method and of its variants, as will be described in the following text, provides the regeneration of the particle sensor in the context of on board diagnosis in a diesel internal combustion engine. In this application, what is important is, in particular, precise and reproducible diagnosis of the particle loading of a soot particle filter (diesel particulate filter, DPF) which is arranged in the exhaust gas section of the diesel internal combustion engine. This is advantageous, in particular, in particle sensors which, as viewed in the flow direction of the exhaust gas, are installed downstream of a particle filter, and it is unclear here whether the particle sensor functions correctly and the exhaust gas only has a low soot proportion to none at all or whether the particle sensor is faulty and the exhaust gas is possibly heavily loaded with soot, however. Furthermore, blockages in the protective tube or even deliberate removal of the particle filter can be diagnosed. Faulty measurements can likewise be detected as such and either a corresponding correction of the measured data can be performed or, for example, an exchange of the sensor can be initiated. Considerably improved operational reliability in comparison with the prior art can therefore be achieved. 
         [0016]    The method according to the invention utilizes the fact that the typically prevailing conditions in the exhaust gas section which are generally dependent on the operating point, such as temperature, pressure and volumetric flow, are known, and a prognosis about the actual sensor temperature and therefore about the heat output P H  which is to be expected or is required can be produced herefrom, in a modeled manner from an inflow into the protective tube which surrounds the particle sensor and from the thermal coupling to the sensor element in the particle sensor. This relates both to an unheated sensor (purely passive incident flow) and to a heated sensor, the temperature response of which is then not only dependent on the surrounding conditions, but rather, in particular, also on the inherent heating. In principle, this method can also be used in other exhaust gas probes with a heating device. 
         [0017]    There is provision in one method variant for the measured heat output P H  to be determined directly from a heating current I H  and a heating voltage U H  applied to the heating element in accordance with P H =U H *I H  or from their effective values, which is advantageous in the case of pulse width modulated supply voltages, or for the measured heat output P H  to be determined from a replacement variable. 
         [0018]    If the heating current I H  cannot be determined or can be determined only with increased outlay, the heating voltage U H  can be used as replacement variable in one advantageous method variant. 
         [0019]    There is provision in another method variant for a heat output which can be provided to be used as replacement variable, which heat output is determined from an available supply voltage U Bat  (battery voltage) and a maximum possible heating duty factor TV max . 
         [0020]    In particular in the case of diagnoses which are intended to detect a defect of the heating element, it can be advantageous if, in a first diagnosis step, a temperature measuring device which is usually configured as a separate temperature dependent resistance element is used in the exhaust gas probe to detect directly whether a setpoint temperature to be reached has been reached, and it is determined in a second diagnosis step whether, if this is the case, a deviation has been caused as a consequence of unfavorable operating conditions. Deviations of this type can be, for example, an excessively high exhaust gas speed or an excessively low supply voltage. If deviations of this type are not detected, a defect in the heating element has to be assumed, which can then be reported as a defect to a superordinate control unit, for example to the engine controller. 
         [0021]    There can be provision in one method variant for further variables which go beyond the heat output P H  determined in a modeled manner to be determined in a modeled manner and to be used for a comparison. This can be, in particular, a modeled temperature which is determined from the heat output P H  determined in a modeled manner and is compared with a measured temperature. Furthermore, a comparison is also possible of an exhaust gas speed which is calculated from the heat output P H  with an exhaust gas speed which is determined by the engine controller, based on a signal of an air mass flow rate sensor which is installed within the air induction means of the internal combustion engine. 
         [0022]    Since the sensor element of an exhaust gas probe, for example of a particle filter, has a thermal inertia, delays will occur between the occurrence of changes in the input variables, such as heat output, exhaust gas speed, exhaust gas temperature, etc., and the occurrence of changes resulting therefrom in the output variables, such as the sensor element temperature for example. This has to be taken into consideration correspondingly in the model formation or in the diagnosis applications. There is therefore provision in one preferred method variant for input variables for the measured parameters for the gas stream to be subjected first of all to a filtering operation and to be subsequently processed further for determining the modeled heat output. Above all, the use of a 1st order low pass filter is possible here. 
         [0023]    There can be provision in a further method variant for temporal changes of the variables which are determined in a modeled manner to be compared, instead of the absolute values, with the measured variables or their changes and to be evaluated. It can be favorable, in particular, here to observe changes in a relatively short time period. This can be advantageous if it is known that certain disturbance variables are variable only over longer time periods than those considered. In this case, said disruption variables could be assumed to be constant. Thus, for example, the temperature of the exhaust gas and exhaust gas pipe of an internal combustion engine can be considered to be variable only relatively slowly if the installation location of the particle sensor is situated relatively far to the back, in particular behind the particle filter of the exhaust gas purification system in the exhaust gas section. 
         [0024]    One method variant provides, during the evaluation, for an observed change in the heat output to be compared with a change in the measured or calculated parameters for the gas stream, the ratio of change in the heat output and change in the measured or calculated parameters for the gas stream being compared with a predefined threshold value or a variable threshold value which is a function of the parameter or the parameters which defines/define the gas stream. A dynamic adaptation of the evaluation can therefore be realized, it being possible for different threshold values to be calculated, for example, in different operating states, which results in a more precise diagnosis function. For example, the change in the heat output P H  which is defined via the heating voltage U H  can be compared with the change in the exhaust gas speed, it being possible for the ratio to be compared with a threshold value which is a function of the exhaust gas speed or the change therein. This threshold value can be stored in the sensor control unit, for example, as a function of the exhaust gas speed or the change therein in the form of a characteristic curve. 
         [0025]    It is advantageous with regard to a simple evaluation if the diagnosis is carried out during a phase at a constantly controlled temperature or during the regeneration of the exhaust gas probe which is configured as a particle sensor during a heating ramp for achieving a free burning temperature required for soot elimination or in the case of deviations of the measured actual temperature from the expected temperature setpoint value. 
         [0026]    One application of the method, as it has been described in the preceding text with its method variants, envisages the use for alternative diagnosis uses, another parameter which denotes the correct operation of the exhaust gas probe being determined in a modeled manner and being compared with a measured value for this parameter instead of a comparison of the modeled heat output P H  with a value which represents the measured heat output P H . A parameter of this type can be the heating resistance R H , for example. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    In the following text, the invention will be explained in greater detail using one exemplary embodiment which is shown in the figures, in which: 
           [0028]      FIG. 1  shows, in a diagrammatic illustration, the technical surroundings, in which the method can be applied, and 
           [0029]      FIG. 2  diagrammatically shows an exhaust gas probe which is configured as a particle sensor, in an exploded illustration. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  diagrammatically shows the technical surroundings, in which the method according to the invention can be applied. An internal combustion engine  10  which can be configured as a diesel engine is fed combustion air via an air feed means  11 . Here, the air quantity of the combustion air can be determined by means of an air mass flow rate meter  12  in the air feed means  11 . The air quantity can be used in a correction of an accretion probability of particles present in the exhaust gas of the internal combustion engine  10 . The exhaust gas of the internal combustion engine  10  is discharged via an exhaust gas section  17 , in which an exhaust gas purification system  16  is arranged. Said exhaust gas purification system  16  can be configured as a diesel particle filter. Furthermore, an exhaust gas probe  15  which is configured as a lambda probe and an exhaust gas probe  20  which is configured as a particle sensor are arranged in the exhaust gas section  17 , the signals of which exhaust gas probes  15 ,  20  are fed to an engine controller  14 . Furthermore, the engine controller  14  is connected to the air mass flow rate meter  12  and, on the basis of the data which are fed to it, determines a fuel quantity which can be fed to the internal combustion engine  10  via a fuel metering means  13 . 
         [0031]    In the example shown, the particle sensor (exhaust gas probe  20 ) is arranged behind the exhaust gas purification system  16  in the flow direction of the exhaust gas, which affords advantages with regard to a homogenization of the exhaust gas stream at this point and is the case, in particular, when used in the context of on board diagnosis. By way of the apparatuses shown, an observation of the particle emissions of the internal combustion engine  10  and a prognosis of the loading of the exhaust gas purification system  16  which is configured as a diesel particulate filter (DPF) are possible. 
         [0032]      FIG. 2  shows, in a diagrammatic illustration, an exhaust gas probe  20  which is configured as a particle sensor, in accordance with the prior art, in an exploded illustration. 
         [0033]    A first electrode  22  and a second electrode  23  are applied on insulating carrier layers  21 , for example made from aluminum oxide. The electrodes  22 ,  23  are configured in the form of two interdigital comb electrodes which engage into one another. A first connection  24  and a second connection  25  are provided at the front-side ends of the electrodes  22 ,  23 , via which connections  24 ,  25  the electrodes  22 ,  23  can be connected for voltage supply and for carrying out the measurement with a sensor control unit (not shown). In the exemplary embodiment, the electrodes  22 ,  23  and the uppermost insulation carrier layer  21 , on which the electrodes  22 ,  23  are situated, are covered with a protective layer  27 . This optional protective layer  27  protects the electrodes  22 ,  23  against corrosion at the usually prevailing high operating temperatures of the particle sensor. In the present exemplary embodiment, said protective layer is produced from a material with a low conductivity, but can also be made from an insulator. In addition, in the example shown, a heating element  26  is integrated between the insulating carrier layers  21 , which heating element  26  is connected via additional connections to the sensor control unit and can be loaded at least temporarily with a heating voltage U H , with the result that a heating current I H  can flow. Firstly the heating element  26  itself or a temperature sensor element which is integrated separately into the particle sensor (for example, as a Pt100 resistance track or as an NTC or PTC ceramic sensor element) can be used for the temperature measurement. 
         [0034]    If a particle sensor of this type is operated in a gas stream which conducts in a particle  28 , for example in an exhaust gas channel of a diesel engine or a combustion plant, particles  28  from the gas stream accrete on the particle sensor. In the case of the diesel engine, the particles  28  are soot particles with a corresponding electric conductivity. Here, the accretion rate of the particles  28  on the particle sensor also depends, in addition to the particle concentration in the exhaust gas, on the voltage, inter alia, which prevails at the electrodes  22 ,  23 . The loading with particles can be determined, for example, by means of a resistance or impedance measurement at the electrodes  22 ,  23 . If the particle sensor is covered with a layer of particles  28  to such an extent that additionally accreted particles  28  do not lead to an additional change in the resistance or the impedance of the particle sensor, the particle sensor is regenerated within a regeneration phase. To this end, the particle sensor is heated with the aid of the heating element  26  to such an extent that the attached particles  28  burn. 
         [0035]    Here, the method according to the invention provides in one exemplary embodiment for the absolute electric heat output to be compared with a predicted heat output for the self-diagnosis of the particle filter, the predicted heat output being calculated from the boundary conditions in the exhaust gas section  17  of the internal combustion engine  10 . If the two heat outputs do not coincide within predefined limits, a defect of the particle sensor is derived or a faulty condition in the exhaust gas section  17  is deduced. 
         [0036]    In one case, a comparison is carried out of the maximum heating voltage U H  which can be demanded and is calculated from an available supply voltage U Bat  and a maximum possible heating duty factor TV max  according to the relationship U H =U Bat *√TV max . In the case of a setpoint value deviation at excessively low temperatures, this is the variable, with which the heat output P H  can be quantified. A heating voltage which is required under the boundary conditions can be determined from a characteristic curve which is a function of the exhaust gas volumetric flow. If the heating voltage which can be demanded is greater than the required heating voltage, it can be deduced that the heating element  26  is defective. 
         [0037]    Here, it can be advantageous with regard to the diagnosis of a possible sensor removal and possible protective pipe blockage also to track the change in the heat output as a reaction to changes in the exhaust gas speed and to detect whether the heat output changes which is necessary to maintain a constant temperature. This is favorable, in particular, when the changes in the exhaust gas speed takes place within a short observation time period, typically &lt;30 s. In this case, effects of changes in the exhaust gas temperature and the tube wall temperature of the particle sensor can be considered to be low, since the stated variables can change only over relatively long time periods. 
         [0038]    Here, defect detection can be realized in the following steps. Within a predefined time interval, the minimum and maximum exhaust gas speed w min  and w max  are determined. The change Δw=w max −w min  is calculated from this. The associated heat output which is quantified here by an effective heating voltage U H  is likewise detected here and the associated change ΔU H  is calculated. If the change Δw exceeds a predefined threshold, a diagnosis can be carried out. Otherwise, the diagnosis is considered to have not been carried out. Furthermore, it is as a rule necessary not to carry out the diagnosis if the exhaust gas temperature exceeds a predefined threshold value since, in the case of an equalization of exhaust gas temperature and sensor element temperature, no heat output changes are to be expected even in the case of an intact sensor. 
         [0039]    The observed heat output change ΔU H  is subjected to a plausibility check. This can take place, for example, by comparison of the measured ratio ΔU H /Δw with a predefined threshold value (ΔU H /Δw) min . If the threshold is exceeded, a defect is detected. Here, alternative comparison methods are also conceivable, such as the comparison of the measured heat output change ΔU H  with a Δw dependent characteristic curve ΔU H,min =f(Δw) of threshold values. Here, as already described above, filtered values for U H  and w can be used. 
         [0040]    In addition, a widening of the function of the particle sensor is conceivable with a corresponding measuring strategy. Said particle sensor is after all usually equipped with a temperature measuring function. However, the volumetric flow is not detected. It is now known that the thermal coupling of the sensor element to the exhaust gas has a dependence on the volumetric flow. This dependence can be used to deduce the actual volumetric flow as a function of the temperature and the change in the temperature. A model understanding for the protective pipe throughflow as a function of the volumetric flow is necessary for this. Here, the inclusion of an additional temperature signal from a further temperature sensor can be helpful with regard to improved measuring accuracy.