Abstract:
A method for regenerating a particle sensor, which comprises a ceramic base body, in the exhaust gas duct of an internal combustion engine for driving a motor vehicle, wherein a particle loading of the particle sensor is determined by applying an electrical voltage between at least two electrodes with interdigital arrangement, a temperature of the particle sensor is determined with a temperature sensor mounted to the ceramic base body or from the electrical resistance of a heating element and said particle sensor is regenerated by means of heating with the electrical heating element.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for regenerating a particle sensor, which comprises a ceramic base body, in the exhaust gas duct of an internal combustion engine for driving a motor vehicle, wherein a particle load of the particle sensor is determined by applying an electric voltage between at least two electrodes with interdigital arrangement, a temperature of the particle sensor is determined with a temperature sensor or meander mounted to the ceramic base body or from the electrical resistance of a heating element, and the particle sensor is regenerated by heating with the electrical heating element. 
     The invention further relates to a device for regenerating a particle sensor, which comprises a ceramic base body, in the exhaust gas duct of an internal combustion engine, wherein the particle sensor comprises at least two electrodes with interdigital arrangement for determining a particle load, a temperature sensor or meander mounted to the ceramic base body for determining a sensor temperature of the particle sensor and an electrical heating element for regenerating said particle sensor, and wherein an engine management system is provided for controlling the internal combustion engine and for acquiring and evaluating output signals of the particle sensor. 
     Legislative regulations stipulate the monitoring of the composition of the exhaust gas of internal combustion engines for compliance to limit values. Particle sensors are, for example, used to monitor particulate emissions of internal combustion engines and for the on-board diagnostics (OBD) within the scope of a functional monitoring of particle filters. In this regard, collecting, resistive particle sensors (particulate matter sensors or PM sensors) are known which evaluate a change in the properties of an interdigital electrode structure on the basis of particle deposits. Two or a plurality of electrodes can be provided which engage in one another in a comb-like manner. Downstream of the diesel particle filter, the exhaust gas of the internal combustion engine is thereby guided past the electrode structure by means of a double-walled protective pipe construction. Due to an increasing number of particles deposited on the particle sensor, the electrodes are bridged by the particles, which results in a decrease in electrical resistance with increasing particle deposit, in a decrease in impedance or in a change in a characteristic variable related to the resistance or the impedance, such as a voltage and/or a current. For evaluation purposes, a threshold value, for example a measurement current between the electrodes, is generally defined and the time up to achieving the threshold value is used as the measurement for the deposited particle quantity. The rate of change of a signal can also alternatively be evaluated during the particle deposition. 
     The German patent publication DE 101 33 384 A1 discloses a resistive particle sensor. The particle sensor is constructed from two comb-like electrodes which engage in one another and are at least partially covered by a capturing sleeve. If particles are deposited on the particle sensor from a stream of gas, this leads to a change in the impedance of the particle sensor which can be evaluated, and from which the quantity of deposited particles and therefore the quantity of particles carried along in the exhaust gas can be determined. 
     If the particle sensor is fully loaded, the deposited particles are burned off in a regeneration phase with the aid of a heating element integrated in the particle sensor. To this end, the ceramic base body of the particle sensor is heated up to a high temperature, whereby said base body is however susceptible to damage due to regional thermal shock which can result from adherent or impacting water droplets. A regeneration of the particle sensor can therefore only be initiated if it can be assumed that no water can reach the particle sensor. To this end, a heat quantity calculation is carried out in an engine management system associated with the internal combustion engine, and a dew point release is provided if no water is presumably to be expected. The underlying assumptions are thereby based on a typical maximum quantity of water in the exhaust gas system which typically occurs during cold starting. 
     Such a consideration does not include a case like driving through water in which the outlet of the exhaust gas system can lie below the water line and water can penetrate into the exhaust gas system or in which the exhaust gas system can be so greatly cooled down from the outside that the dew point of the gas mixture situated therein is undershot. Ingressed or condensed water is in fact expelled again by means of exhaust gases. After a detected water crossing, the point must however be initially reached where conditions again prevail in the exhaust gas system which are required for a normal dew point release via the thereby utilized pipe wall temperature models and heat quantity integrals in the engine management system. 
     SUMMARY OF THE INVENTION 
     It is therefore the task of the invention to provide a method which also enables a regeneration of a particle sensor without the risk of damage by temperature shock even, e.g., after a water crossing operation. 
     It is furthermore the aim of the invention to provide a corresponding device for carrying out the method. 
     The aim relating to the method is met in that, after the motor vehicle has driven through water with the heating element switched off and the electrodes free of voltage, the temperature of the particle sensor, an exhaust gas temperature and/or a pipe wall temperature are determined as a release condition of the particle sensor for regeneration or for a measurement phase and in that the release condition is considered to be fulfilled if the temperature exceeds a predetermined release temperature. In order to prevent damage to the ceramic base body of the particle sensor due to large regional temperature differences when water strikes the hot particle sensor, the temperature is determined on the unheated particle sensor and compared to the boiling point of water. In the event that the sensor temperature lies above the boiling point temperature in this operating mode, it can be assumed that no water is adhering to the sensor. If the sensor temperature is still below or in the range of the boiling temperature after a water crossing operation of the vehicle in which water can penetrate into the exhaust gas system or in which the exhaust gas system can be externally cooled down so much by the water that the dew point of the gas mixture situated in the exhaust gas system is undershot, an immediate protective heating of the particle sensor can initially be omitted. During protective heating of the particle sensor, the heater is operated at reduced power in order to evaporate traces of water. If large quantities of water are, however, present in the system after a water crossing operation, protective heating is not yet useful. If the interdigital electrodes which determine a particle coating are switched free of voltage in this operating mode, damage to said electrodes by electrolytic processes is prevented. 
     If the release condition is combined with a dew point release in order to release the particle sensor or if the release condition initiates a dew point release, the protection of the particle sensor from damage can be further improved. A heat quantity calculation is carried out during the dew point release with the aim of ensuring that water can no longer be present at the sensor position under typical operating conditions. This typically includes the water quantity in the system during cold starting, however not water that has ingressed into the exhaust gas system after driving through water. 
     An improvement in enabling a regeneration or measurement phase of the particle sensor is achieved in that, in order to release the particle sensor, the release condition from the temperature of the particle sensor is combined with a release from the detection of a water crossing by the vehicle. 
     One embodiment of the method according to the invention provides that a profile of the temperature of the particle sensor, the exhaust gas temperature and/or the pipe wall temperature are acquired after a water crossing operation of the vehicle and that the release condition is considered fulfilled after a detected temperature plateau if the predetermined release temperature has been exceeded. When analyzing the temperature profile it can be suggested from the occurrence of the temperature plateau that water is evaporating in the system in this phase and holds the temperature approximately constant due to the heat quantity thereby absorbed. If an end of the plateau phase is determined, this means that water located in the system has substantially evaporated and that the release procedures that are typically provided for releasing a regeneration or a measurement phase of the particle sensor can be started. 
     The inventive operation is preferably carried out in a control device in structural proximity to the particle sensor and provision is made for the heating element and the voltage at the electrodes to be switched off after an on-site control device of the particle sensor detects the vehicle is being driven through water and for a switch-off signal to be sent to a higher-level engine management system. This outsourcing of a part of the operation simplifies the system. The temperature analysis is carried out in the higher-level engine control device with the aid of the temperature information transmitted by the particle sensor. 
     The aim relating to the device is met in that a circuit or a program sequence for detecting a water crossing operation, for evaluating the sensor temperature, an exhaust gas temperature and/or a pipe wall temperature is provided in the engine management system, in that, after a water crossing operation, a release condition of the particle sensor for regeneration or for a measurement phase is fulfilled if the sensor temperature, the exhaust gas temperature and/or the pipe wall temperature exceed a predetermined release temperature. If the release condition is not fulfilled and thus there is possibly water on the particle sensor, a measurement phase is not yet useful because the resistance between the interdigital electrodes can also be caused by water instead of from sooty particles. The aforementioned pipe wall is the wall of the protective pipe around the particle sensor which serves to protect the particle sensor and also to carry the exhaust gas and improve the deposition of particles on the electrodes. The device according to the invention improves the protection of the particle sensor from cracks due to high temperature differences at the ceramic components thereof as said temperature differences may occur as a result of water droplets adhering to or striking on the particle sensor after a water crossing operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained below in detail with the aid of an exemplary embodiment depicted in the figures. In the drawings: 
         FIG. 1  shows, in a schematic depiction, the technical environment in which the method can be used; 
         FIG. 2  shows an exhaust gas sensor embodied as a particle sensor; and 
         FIG. 3  shows a time diagram of temperature profiles in the exhaust gas duct during a water crossing. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows schematically the technical environment, in which the method according to the invention can be used. An internal combustion engine  10 , which can be embodied as a diesel engine, is provided combustion air via an air inlet  11 . In so doing, the air volume of the combustion air can be determined using an air flow meter  12  in the air inlet  11 . The air volume can be used during a correction of a deposition probability of particles present in the exhaust gas of the internal combustion engine  10 . The air volume being supplied furthermore serves to determine exhaust gas parameters, such as exhaust gas quantity, a volume flow or a speed. The exhaust gas of the internal combustion engine  10  is discharged via an exhaust gas tract  17  in which an exhaust gas emission control system  16  is disposed. Said exhaust gas emission control system  16  is embodied as a diesel particle filter. In addition, a lambda probe  15  and a particle sensor  20  are disposed in the exhaust gas tract  17 , the signals of which are supplied to an engine management system  14 . The engine management system  14  is furthermore connected to the air flow meter  12  and determines a fuel quantity on the basis of the data supplied thereto, which fuel quantity can be supplied via a fuel metering  13  of the internal combustion engine  10 . In addition, the temperature of the particle sensor  20  that is required for carrying out the inventive method is determined in the engine management system  14 , and release conditions for a regeneration of the particle sensor  20  or for the release of a measure phase of the particle sensor  20  are ascertained. 
       FIG. 2  shows the particle sensor  20  in a schematic depiction. A first electrode  22  and a second electrode  23  are mounted to insulating substrates  21  which form a ceramic base body, for example consisting of aluminum oxide. The electrodes  22 ,  23  are embodied in the form of two mutually engaging comb electrodes (interdigital electrodes). A first terminal  24  and a second terminal  25  are provided on the end faces of the electrodes  22 ,  23 . The electrodes  22 ,  23  are connected via said terminals to the engine management system  14  for the purpose of supplying voltage and for carrying out the measurement. In the exemplary embodiment, the electrodes  22 ,  23  and the uppermost insulating substrate  21 , on which the electrodes  22 ,  23  are located, are covered with a protective layer  27 . This optional protective layer  27  protects the electrodes  22 ,  23  from corrosion during the high operating temperatures of the particle sensor which prevail for the most part. In the example depicted, a heating element  26 , which is connected via additional terminals to the engine management system  14  and can at least periodically be stressed by a heating voltage UH, is additionally integrated between the insulating substrates  21 ; thus enabling a heating current IH to flow. In order to measure the temperature, the heating element  26  itself can on the one hand be used or a temperature sensor element separately integrated into the particle sensor (e.g. as a meander-shaped Pt 100 resistance sheet or as a NTC or PTC ceramic sensor element). 
     If such a particle sensor is operated in a stream of gas carrying particles  28 , for example in the exhaust gas tract  17  of an internal combustion engine  10 , the particles  28  from the stream of gas are deposited on the particle sensor  20 . In the case of a diesel engine, the particles  28  relate to soot particles. In this connection, the deposition rate of the particles  28  on the particle sensor  20  is not only a function of the particle concentration in the exhaust gas but also inter alia of the voltage which is applied to the electrodes  22 ,  23 . The loading with particles can, for example, be determined by means of a resistance or impedance measurement at the electrodes  22 ,  23  because the particles  28  are electrically conductive. If the particle sensor  20  is loaded with a layer of particles to the extent that particles  28  being additionally deposited do not lead to a further change in the resistance or the impedance of the particle sensor, the particle sensor  20  is then regenerated in a regeneration phase. To this end, the particle sensor  20  is heated with the aid of the heating element  26  to the extent that the deposited particles  28  are burned off. 
     In  FIG. 3 , temperature profiles in the exhaust gas tract  17  are shown in a time diagram  30  plotted over a time axis  38  and a signal axis  31 . A temperature sensor on the particle sensor  20  emits a sensor temperature  32 . In addition, the profile of an exhaust gas temperature  34  and a pipe wall temperature  35  are depicted. The pipe wall temperature  35  is the temperature of a protective pipe surrounding the particle sensor  20 . At the beginning of the water crossing, the sensor temperature  32 , the exhaust gas temperature  34  and the pipe wall temperature  35  drop sharply because water penetrates into the exhaust gas tract  17  and cools the components. After completion of the water crossing, the temperature signals rise again. In this respect, a temperature plateau typically emerges from this condition, wherein the water evaporates at a constant boiling temperature. The sensor temperature  32 , the exhaust gas temperature  34  and the pipe wall temperature  35  subsequently continue to increase. After a detected water crossing state, a conclusion can be drawn about the current profile of the pipe wall temperature  35  and the exhaust gas temperature  34  by the analysis of the sensor temperature  32  when the heating element  26  is switched off. This is the case because the sensor temperature  32  lies between said two aforementioned temperatures  34 ,  35  in an application specific manner. It can be inferred from the termination of the temperature plateau  37  that the water has evaporated and the particle sensor  20  can be heated by means of the heating element  26  without risk of damage thereto. To this end, a release temperature  36  is predetermined, a release condition being generated in the engine management system  14  in the event of said release temperature being exceeded. This release condition can be logically linked to other signals, such as a release for regeneration or detection of the completion of a water crossing. All together it is therefore possible by means of the inventive method in contrast to the prior art to define the point in time at the installation position of the particle sensor  20 , at which point in time the water is reduced to a sufficiently small amount after a water crossing operation so that a sensor regeneration or a sensor measurement phase can be released.