Abstract:
The invention relates to a method and a device, in particular a control and evaluation unit, for operating at least one exhaust gas sensor for monitoring the functionality of an emission control system in the exhaust tract of an internal combustion engine, wherein the exhaust gas sensor is operated at least intermittently at high temperatures and has a thermal shock sensitivity inherent to the design, and in which a heating phase can be implemented at least intermittently prior to a regeneration phase or prior to a measuring operation phase, wherein a clearly lower temperature is set in this heating phase in comparison to the regeneration temperature or the measuring operation temperature. According to the invention, the function for dew point recognition and re-release of a dew point end is adaptively implemented and influenced by at least one water detection criterion or at least one flood detection criterion. By this means, an improved dew point detection is achieved after driving through water, as a result of a significant reduction of tolerances and an earlier release of the dew point end for the exhaust gas sensor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method and a device, in particular a control-and-evaluation unit, for operating at least one exhaust-gas sensor for monitoring the functionality of an exhaust emission control system in the exhaust train of an internal-combustion engine, wherein the exhaust-gas sensor is operated at high temperatures at least temporarily and, due to its type of construction, exhibits a sensitivity to thermal shock, and in which prior to a regeneration phase or prior to a measuring-mode phase a heating phase can be carried out at least temporarily, wherein in this heating phase a distinctly lower temperature is adjusted in comparison with the regeneration temperature or measuring-mode phase. 
         [0002]    The invention further relates to a device, in particular a control-and-evaluation unit for operating exhaust-gas sensors and for implementing the method as claimed in the invention. 
         [0003]    Exhaust-gas sensors—such as, for example, lambda probes, particle sensors, HC sensors or nitrogen-oxide sensors—as claimed in the state of the art are based on ceramic sensor elements that in operation are heated at least temporarily. 
         [0004]    Particle sensors (PM) are employed nowadays, for example, for monitoring the particulate-matter (soot) emission of internal-combustion engines and for on-board diagnosis (OBD), for example for functional monitoring of particle filters, for example of a diesel particle filter (DPF). Such a resistive particle sensor is described in DE 101 33 384 A1. The particle sensor has been constructed from two intermeshing, comb-like electrodes (interdigital electrodes IDE) which have been covered at least partly by a trapping sleeve. If particles from a stream of gas are deposited on the particle sensor, this results in an evaluable change in the impedance of the particle sensor, from which the quantity of accreted particles—and consequently the quantity of particles entrained in the exhaust gas—can be inferred. 
         [0005]    If the particle sensor is fully laden, the accreted particles are burnt in a regeneration phase with the aid of a heating element integrated within the particle sensor. For this purpose, the ceramic of the sensor element is heated to high temperatures, ordinarily to &gt;600° C. As a rule, the heating element exhibits a temperature-measuring structure (meander) with which the temperature of the particle sensor can be monitored and the heating power during this regeneration phase can be regulated. 
         [0006]    In this regeneration phase the sensor element reacts sensitively to great local changes of temperature or to a thermal shock such as may arise as a result of incident water or drops of water. A thermal shock of such a type can lead to cracks in the sensor element. Similar problems also arise in the case of the other aforementioned exhaust-gas sensors. Therefore a sensor regeneration is demanded by the engine control unit only when, as claimed in a heat-quantity calculation in the engine control unit, no more water can be present at the installation position of the sensor. 
         [0007]    Furthermore, a loading with water must be prevented from occurring while the temperature of the sensor element of exhaust-gas sensors of such a type is greater than a certain threshold-value temperature, typically about 200° C. Therefore, particularly after a cold start, as long as condensate may still be located in the exhaust train of an internal-combustion engine, operation with heating at a temperature &gt;200° C. of the exhaust-gas sensors takes place only after a certain time in which it may be assumed that, in this time, all the water has either evaporated or been discharged, in the form of droplets, from the exhaust train by virtue of surges of gas in automotive operation. This moment is typically designated as the dew-point end (DPE) and depends on many conditions, for which reason this moment has to be determined in the specific application for each type of vehicle. Operation of the exhaust-gas sensors at temperatures &gt;200° C. is then permitted, so long as no condensation of water in the exhaust train occurs in the region of the installation point of the sensor as a consequence of cooling. 
         [0008]    Particle sensors and also other heated exhaust-gas sensors utilize known dew-point detection functions in order to detect whether liquid water is present in the exhaust train (see, for example, document DE 43 00 530 C2 or DE 43 38 342 C2). With these functions it is determined, on the basis of the exceeding of a pipe-wall temperature threshold and on the basis of the exceeding of a threshold quantity of exhaust-gas heat, that no more liquid water can be present at the location of the sensor. The limiting quantity of heat is, in turn, dependent on the pipe-wall temperature that was present at the start of the driving cycle. The temperatures taken as a basis for this—such as the exhaust-gas temperature and the pipe-wall temperature—are model-based in the contemporary practical realization and are reproduced in an exhaust-gas-temperature model. 
         [0009]    As a result of intense loading of the exhaust system with water from outside, for example during fording travel or in the course of launching a boat into water along a slip-ramp, flooding may occur, whereby water penetrates into the exhaust train, as a result of which an intense cooling of the exhaust system may occur. Depending on the configuration of the exhaust system, this cooling cannot be detected by the engine control unit but may result in an exposure to danger as a result of thermal shock to the sensor element of the exhaust-gas sensors. This case has not been covered hitherto in a dating of the dew-point end—that is to say, water remains in the exhaust train, even though the previous DPE-detection function cannot detect this on the basis of limiting quantities of heat and pipe-wall temperatures. If the sensor is regenerated while or after this state has arisen, the risk of a thermal shock is particularly high. 
         [0010]    Furthermore, also during fording travel the exhaust pipe may be cooled in such a manner that a condensation of liquid water occurs. The sensor is then loaded with drops of liquid. The dew-point detection function is frequently based on model temperatures (see, for example, DE 10 2006 010 094 A1) which, in turn, are based on temperature measurements that were carried out using sensors which, however, have been arranged upstream of the particle sensor and the aforementioned exhaust-gas sensors. Since these temperature sensors remain unaffected by the fording state, such a fording state cannot be detected. If the particle sensor is regenerated, for example, damage to the sensor may occur as a result of thermal shock, even though a dew-point end was detected. 
         [0011]    Furthermore, an increase may occur in the IDE current signal, which is caused by the conductivity of liquid water. This may be misinterpreted as soot loading of the sensor element. 
         [0012]    In other, as yet unpublished, parallel applications by the applicant—inter alia, for example, the application having internal file reference R.351847—fording-detection criteria and also flood-detection criteria are furthermore described. Furthermore, in the likewise as yet unpublished application by the applicant having internal file reference R.347466 a utilization of a combination of criteria for enabling the heating of the exhaust-gas sensor is also described. 
       SUMMARY OF THE INVENTION 
       [0013]    It is an object of this invention to make available an improved method for dew-point detection and also to adapt a re-enabling of the dew-point end to the situation of fording travel. 
         [0014]    It is furthermore an object of the invention to make available an appropriate device for implementing the method, in particular a control-and-evaluation unit. 
         [0015]    The object relating to the method is achieved in that the function of a dew-point detection and the re-enabling of a dew-point end are carried out adaptively and are influenced by at least one fording-detection criterion or at least one flood-detection criterion. By virtue of this adaptation, the dew-point detection can be improved in comparison with the state of the art, since environmental influences that cannot be predicted by modeling, such as those described in the introduction, can also be taken into account. This serves, on the one hand, for enhanced protection of the exhaust-gas sensors incorporated in the exhaust train of the internal-combustion engine, but, on the other hand, also for an earlier resumption, which is thereby possible, of the measuring tasks of the exhaust-gas sensors, or for an earlier implementation of a necessary regeneration phase. 
         [0016]    A particularly advantageous variant of the method provides that temperature quantities of temperature-measuring instruments that have been incorporated within the exhaust-gas sensor or near the exhaust-gas sensor in the exhaust train of the internal-combustion engine are used for the purpose of detecting the dew point, and these temperature quantities are used at least temporarily as substitutional quantities for otherwise modeled temperatures in the course of the dew-point detection. The influence of model tolerances can be reduced by this. This also holds if there is no fording state. For instance, smaller lead-time compensations may be provided with the method, as a result of which an earlier enabling of the dew-point end tends to be possible. This, in turn, allows an earlier enabling of the monitoring and the control/regulation of exhaust-gas aftertreatment systems and of the engine control/regulation. In particular, the determination of the temperature directly in or on the exhaust-gas sensor offers advantages here in situations such as fording travel in comparison with a temperature quantity ascertained by modeling. 
         [0017]    In a preferred variant of the method, initial values of the modeled temperatures in the course of the dew-point detection, which may correspond to cooling curves of parts of the exhaust-gas sensor, are replaced by measured temperatures. Hence, in particular, the environmental influences, particularly those in the case of fording travel, can be taken into account better in the course of the dew-point detection, since the temperature quantities determined by modeling may deviate distinctly from the real temperature conditions, and consequently misinterpretations may occur and, where appropriate, necessary times up until a dew-point end may be calculated incorrectly. 
         [0018]    In this connection it may, in addition, have been stipulated that differing limiting quantities of heat are chosen for the purpose of ensuring a sufficient drying of the exhaust train and of the exhaust-gas sensors incorporated therein, depending on whether the function of dew-point detection is based on model temperature quantities or measured temperature quantities. In the case of model temperature quantities, larger reserves are preset as a rule, in order to take model tolerances into account. 
         [0019]    Furthermore, in a preferred practical variant of the method it may have been stipulated that by way of decision criterion for utilizing a model temperature quantity or a measured temperature quantity a temperature comparison between the model temperature quantity and the measured temperature quantity is carried out after a minimum shutdown time of the internal-combustion engine, and the minimum value of the two temperature quantities is used at this moment as input quantity for the dew-point detection. By this, it is ensured that the more critical, i.e. lower, temperature in the given case is taken into consideration in the course of the dew-point detection, and consequently a sufficiently long drying-phase can be realized. 
         [0020]    In this connection, a variant of the method provides that in the event of the minimum shutdown time being exceeded the measured temperature quantity is used, and in the event of the minimum shutdown time being fallen short of or attained the minimum value of the two temperature quantities is used. 
         [0021]    A further variant of the method provides that for the purpose of re-enabling the dew-point end (DPE) after a detected fording state or flooding state a limiting quantity of heat, adapted to the situation, is preset. Hence additional introductions of water—for example, as a consequence of a more intense formation of condensation by virtue of the cooling of the exhaust train in the case of fording travel, or the penetration of water in the case of flooding—can be taken into account in the course of the re-enabling of the dew-point end. 
         [0022]    If, as an advantageous variant of the method provides, in the case of a detected flooding a higher limiting quantity of heat is preset than in the case of detected fording travel, it can be ensured that a sufficiently good drying of the components in the exhaust train, particularly of the exhaust-gas sensors, can take place. Since, in the case of fording travel without flooding, besides the moisture introduced by the combustion and the induction air merely a more intense formation of condensation may occur but otherwise no additional water is introduced, the limiting quantity of heat can therefore be designed to be smaller in comparison with a detected flooding. Hence it can also be ensured that the DPE enabling can be undertaken earlier. 
         [0023]    With regard to a simplification of this function, it may also have been stipulated that both in the case of a detected flooding or fording travel a merely increased limiting quantity of heat is preset in comparison with a standard application, i.e. a dew-point enabling without the possibility of detection of flooding or fording. In this case the starting-point is a “worst case” examination, without any further differentiation being made. 
         [0024]    Furthermore, it may have been stipulated that after annulment of a detected fording state or flooding state the limiting temperature of the modeled or measured temperature quantity is increased at least temporarily for the dew-point enabling. This may be done in the simplest case by means of an offset. 
         [0025]    The previously described functionality of the dew-point detection and of the enabling of the dew-point end can be applied particularly advantageously in the case of exhaust-gas sensors that have been designed as lambda probes, particle sensors, nitrogen-oxide sensors, sensors for determining a hydrocarbon content in the exhaust gas, and/or other exhaust-gas sensors based on a ceramic sensor element. During their normal measuring mode said sensors exhibit, on the one hand, high operating temperatures or are subjected at least temporarily to high temperatures (&gt;600° C.), as is the case, in particular, for the purpose of regeneration in the case of particle sensors. On the other hand, these sensors possess increased sensitivity to thermal shock, which, particularly in the case of fording travel as a consequence of intensified cooling or even in the case of a flooding of the exhaust-gas sensor, may result in a malfunction, even in irreversible damage. In this connection it is of particular interest that this type of exhaust-gas sensors are sufficiently dried after fording travel accompanied by flooding. This can be ensured with the previously described functionality for adaptive dew-point detection and DPE enabling. In addition, for the purpose of implementing the method it is advantageous that use may be made of temperature-measuring instruments that have been incorporated within these exhaust-gas sensors or at least spatially near them, so that no additional expenditure arises for the specific application. 
         [0026]    A particularly advantageous application of the method arises if, by way of modeled temperature quantity in the case of the particle sensor used as exhaust-gas sensor, a temperature model is used for a pipe-wall temperature, and the signal of a temperature sensor integrated within the sensor element of the particle sensor, of an NTC resistor incorporated in situ on the particle sensor, or of a heating resistor of a heating element integrated within the particle sensor, is used for the measured temperature quantity. In particular, particle sensors exhibit a high sensitivity to thermal shock. Here it is of particular interest that regeneration phases for eliminating soot are only started or enabled if it has been ensured that no residual water is present and/or the sensor has already dried sufficiently. This can be ensured, on the one hand, by the presetting of the limiting quantity of heat, depending on detected fording travel or flooding, and, on the other hand, by the inclusion of measured temperature quantities for the function of dew-point detection, as previously described. These temperature quantities can be determined in the case of the particle sensor, on the one hand, by a measuring meander in the particle sensor, or by evaluating a resistance of the integrated heating element. Ordinarily, these items of temperature information are already available in connection with the regulation of the heating power for the particle sensor. 
         [0027]    The object relating to the device is achieved in that the function of dew-point detection and the re-enabling of a dew-point end can be carried out adaptively in the control-and-evaluation unit and can be influenced by at least one fording-detection criterion or at least one flood-detection criterion, the control-and-evaluation unit exhibiting equipment for implementing the previously described method with its variants. Ordinarily, this equipment is already available in control-and-evaluation units as claimed in the state of the art. It is advantageous in this connection that no change of hardware has to be made in respect of the particle sensor or in respect of the control-and-evaluation unit for the purpose of implementing the method. This additional functionality can be realized exclusively by a software supplement. In this case, the control-and-evaluation unit may be realized as a stand-alone unit which, for example, lies close to the sensor on the particle sensor, or as an integral constituent of a higher-ranking engine controller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The invention will be elucidated in more detail in the following on the basis of an embodiment represented in the figures. Shown are: 
           [0029]      FIG. 1 : in a schematic representation, the technical environment in which the method can be applied and 
           [0030]      FIG. 2 : schematically, a particle sensor in an exploded representation. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 1  shows schematically the technical environment in which the method as claimed in the invention can be applied. An internal-combustion engine  10 —which, for example, may have been realized as a diesel engine—receives combustion air supplied via an air feed  11 . In this case the quantity of combustion air can be determined by means of a mass airflow meter  12  in the air feed  11 . The quantity of air can be used in the course of a correction of a probability of accretion of particles that are present in the exhaust gas of the internal-combustion engine  10 . The exhaust gas of the internal-combustion engine  10  is conducted away via an exhaust train  17  in which an exhaust emission control system  16  is arranged. This exhaust emission control system  16  may have been realized as a diesel particle filter. Furthermore, in the exhaust train  17  an exhaust-gas probe  15  realized as a lambda probe and a particle sensor  20  are arranged, the signals of which are supplied to an engine controller  14  or to a special control-and-evaluation unit  30  (sensor control unit SCU) which may be a constituent of the engine controller  14  or may have been realized as a stand-alone unit, for example close to the particle sensor  20 . The engine controller  14  is furthermore connected to the mass airflow meter  12  and determines, on the basis of the data supplied to it, a quantity of fuel that can be supplied to the internal-combustion engine  10  via a fuel meter  13 . 
         [0032]    The particle sensor  20  may in this case also have been arranged downstream of the exhaust emission control system  16  in the direction of flow of the exhaust gas, which affords advantages with regard to a homogenization of the flow of exhaust gas at this point and is the case, in particular, when employed within the scope of on-board diagnosis. With the devices shown, an observation of the particulate emission of the internal-combustion engine  10  and a prognosis of the loading of the exhaust emission control system  16  taking the form of a diesel particle filter (DPF) is possible. 
         [0033]      FIG. 2  shows, in a schematic representation, a particle sensor  20  corresponding to the state of the art, in an exploded representation. 
         [0034]    On insulating substrates  21  consisting of ceramics, for example consisting of aluminum oxide, an IDE measuring structure  22  in the form of a first electrode and a second electrode has been applied. The electrodes have been realized in the form of two interdigital intermeshing comb electrodes and are designated as IDE electrodes  23  and constitute the actual sensor element. At the front ends of the IDE electrodes  23  the IDE terminals  24  (IDE+ and IDE−) have been provided, via which the IDE electrodes  23  for supplying voltage and for performing the measurement have been connected to the control-and-evaluation unit  30  (not represented in  FIG. 2 ). In addition, in the example shown, a heating element  26  has been integrated between the insulating substrates  21 , which has been connected to the control-and-evaluation unit  30  via additional heating-element terminals  25  (+H, OV). 
         [0035]    For measuring the temperature, a temperature sensor  27  may additionally have been provided in the layered structure of the particle sensor  20 , in which case a temperature-sensor terminal  28  (TM) has additionally been guided out of the particle sensor  20 . By way of temperature sensor  27 , resistive structures consisting of platinum—for example, the meander structure shown—may, for example, find application. Alternatively, at least a part of the structure of the heating element  26  may also be utilized as temperature sensor  27 . 
         [0036]    If such a particle sensor  20  is operated in a stream of gas conducting soot particles  29 —for example, in an exhaust-gas duct of a diesel engine or of a furnace installation—soot particles  29  from the stream of gas are deposited on the particle sensor  20 . These particles possess a certain electrical conductivity. In this connection, besides depending on the particle concentration in the exhaust gas, the rate of deposition of the soot particles  29  onto the particle sensor  20  also depends, inter alia, on the voltage that is applied to the IDE electrodes  23 . By virtue of the applied voltage, an electric field is generated which exerts a corresponding attraction on electrically charged soot particles  29 . By suitable choice of the voltage applied to the IDE electrodes  23 , the rate of deposition of the soot particles  29  can therefore be influenced. 
         [0037]    In the embodiment, the IDE electrodes  23  and the uppermost insulating substrate  21 , on which the IDE electrodes  23  are located, have been covered with a protective layer. This optional protective layer protects the IDE electrodes  23  against corrosion at the generally prevailing high operating temperatures of the particle sensor  20 . In the present embodiment, said protective layer has been produced from a material having low conductivity, but it may also have been manufactured from an insulator. 
         [0038]    Soot particles  29  from the stream of gas have been deposited in the form of a layer on the protective layer. By virtue of the slightly conductive protective layer, the soot particles  29  form a conductive path between the IDE electrodes  23 , so that, depending on the quantity of the deposited soot particles  29 , a change of resistance between the IDE electrodes  23  results. This change can be measured, for example by a constant voltage being applied to the IDE terminals  24  of the IDE electrodes  23  and by the change in the current through the accreted soot particles  29  being determined. If the protective layer has been constructed to be insulating, the deposited soot particles  29  result in a change in the impedance of the particle sensor  20 , which can be evaluated by an appropriate measurement, preferably with an AC voltage. 
         [0039]    If the particle sensor  20  has been covered with a layer of soot particles  29  to such an extent that additionally accreted soot particles  29  do not result in an additional change in the resistance or impedance of the particle sensor  20 , the particle sensor  20  is regenerated within a regeneration phase. For this purpose, the particle sensor  20  is heated with the aid of the heating element  26  to such an extent that the adjacent soot particles  29  burn. This ordinarily happens at temperatures &gt;600° C. 
         [0040]    Prior to a regeneration of the particle sensor  20 , for a certain time a protective heating ahead of the dew-point end (DPE) is carried out, as described in the introduction. 
         [0041]    The method as claimed in the invention provides, on the one hand, that, in comparison with a standard application of a dew-point detection, a temperature measured by a temperature-measuring instrument on the exhaust-gas sensor, in the following example on the particle sensor  20 , is utilized as input quantity for the dew-point detection, wherein the measured temperature is used as substitutional quantity for the pipe-wall temperature modeled hitherto. Hitherto it has been stipulated that an exhaust-gas-temperature model is initialized with starting values for exhaust-gas temperature and pipe-wall temperature. In this case, as claimed in the contemporary state of the art, use is made of a cooling curve of the pipe-wall temperature. This curve calculates the pipe-wall temperature at the end of the shutdown time of the internal-combustion engine  10  from the temperature values immediately prior to the shutdown time and from the duration of the shutdown time on the assumption of an exponential dependence on time. However, as mentioned in the introduction, this model cannot take environmental influences into account at all, or can only take them into account inadequately. Therefore it may have been stipulated that the initial values for pipe-wall temperatures from cooling curves are replaced by measured temperature values. 
         [0042]    Furthermore, the pipe-wall temperature modeled hitherto enters into the calculation of a limiting quantity of heat. This may also use the temperature of the measuring instrument of the particle sensor  20 . In this connection it may be appropriate to switch between alternative limiting quantities of heat. In this case, a set of limiting quantities of heat is utilized if measured temperatures on the basis of the temperature-measuring instrument are being utilized. A second set is utilized when model temperatures are being employed. In this connection, larger reserves can be allowed for, as a rule, in order to take model tolerances into account. 
         [0043]    As decision criterion for the utilization of the temperature-measuring instrument instead of the model values, a variety of approaches may be pursued. After a minimum shutdown time of the internal-combustion engine  10  has been attained, it may be assumed that the temperature of the sensor element has been assimilated to the pipe-wall temperature. The temperature can consequently be influenced not only by a large temperature difference between gas temperature and pipe-wall temperature, by a past sensor regeneration or by a protective-heating state. Furthermore, it may have been stipulated that a minimum value is derived from the temperature of the measuring instrument of the particle sensor  20  and the model temperature. An appropriate combination of the two aforementioned values may also be used. In the event of a minimum shutdown time being exceeded, the temperature of the temperature-measuring instrument is regarded as valid in every case. In the case where the minimum shutdown time is fallen short of or is just attained, the minimum value of the two temperatures is selected. 
         [0044]    As temperature-measuring instrument may be resistance-measuring elements, for example the resistive meander of the particle sensor  20  described in  FIG. 2 , which ordinarily exhibits a PTC characteristic, or an NTC measuring resistor incorporated in situ, or a heating resistor of the heating element  26  of the particle sensor  20 . 
         [0045]    With regard to a re-enabling of the dew-point end or a withdrawal of a dew-point detection of an exhaust-gas sensor after a fording state, it has furthermore been stipulated that the re-enabling of the dew-point end or the withdrawal of the dew-point detection is adapted to the situation of fording travel. 
         [0046]    In the fording state, the exhaust-gas sensor is acted upon by water that has penetrated into the exhaust train or by water that has condensed as a consequence of external cooling of the exhaust train, and must not be regenerated without prior drying. If it is detected by a functionality that a fording state has arisen, a dew-point undershoot is detected and a sensor regeneration or a particle measurement at the particle sensor  20  is prevented. In this case, conditions adapted to the “fording state” situation apply to the re-enabling of the dew-point end. 
         [0047]    For the purpose of enabling, a limiting quantity of heat adapted to the fording situation is preset which has to be conducted past the sensor together with the exhaust gas, so that said sensor is considered to have been dried. In this case it is possible to differentiate between a flooding of the exhaust system and fording travel—that is to say, transit through water without flooding of the exhaust train. In the case of a flooding of the exhaust train, this limiting quantity of heat is chosen to be higher than in the case of condensation by external cooling as a consequence of fording travel without flooding, since, in the first case, by virtue of the quantity of water introduced a greater quantity of heat is required for an enabling of the dew-point end in order to dry the components in the exhaust train than would be necessary without flooding of the exhaust train. In the case of fording travel without flooding, besides the water stemming from the combustion or from the induction air no additional water is introduced. Therefore the requisite limiting quantity of heat is smaller in this case. 
         [0048]    In a simplified function of the re-enabling of the dew-point end or of the withdrawal of the dew-point detection, this differentiation can also be dispensed with. In this case, the starting-point is a “worst case” examination, wherein a larger limiting quantity of heat is chosen in comparison with a standard application as claimed in the state of the art. 
         [0049]    If a detected fording state is annulled, it may furthermore have been stipulated that the limiting temperature for the re-enabling of the dew-point end is increased via the pipe-wall temperature, in order to take account of the fact that the existing functionality comes from excessive pipe-wall temperatures. In the simplest case this can be done by an applicable offset in the pipe-wall temperature. 
         [0050]    The previously described functionality has preferably been implemented as a software module in an in-situ control unit of the particle sensor  20 —that is to say, in a control-and-evaluation unit  30  of the particle sensor  20 . But it may also be an integral constituent of the higher-ranking engine controller  14  (cf.  FIG. 1 ). 
         [0051]    In principle, the described functionality may also be applied to other exhaust-gas sensors in which a correct dew-point detection is likewise important for the functionality of the exhaust-gas sensors and as protection against damage as a consequence of thermal shock. Besides the previously described particle sensors  20 , exhaust-gas sensors of such a type may also be, for example, lambda probes, HC sensors for measuring hydrocarbon concentrations, or nitrogen-oxide sensors, which, as claimed in the state of the art, are based on ceramic sensor elements that in operation are heated at least temporarily.