Patent Publication Number: US-2006005534-A1

Title: Method for operating a particulate filter disposed in an exhaust-gas region of an internal combustion engine and device for implementing the method

Description:
BACKGROUND INFORMATION  
      German Patent Application No. DE 199 06 287 describes a method and a device for controlling an internal combustion engine, in whose exhaust-gas region an exhaust-gas treatment device is arranged that includes a particulate filter which holds back the particulates contained in the exhaust gas. For proper operation of the particulate filter, it is necessary to know the particulate load state, which may be determined indirectly based on the differential pressure occurring at the particulate filter.  
      German Patent Application No. DE 102 48 431 describes a method for ascertaining the particulate-filter load state, in which the flow resistance of the exhaust gases is utilized as a measure for the particulate load state. The flow resistance is ascertained from the differential pressure occurring at the particulate filter and the exhaust-gas volumetric flow. Taking into consideration the exhaust-gas temperature, it is possible to obtain the exhaust-gas volumetric flow from the exhaust-gas mass flow rate which can be at least approximately calculated from at least one known operating characteristic of the internal combustion engine, e.g., from an air signal provided by an air sensor. The differential pressure occurring at the particulate filter is ascertained from a pressure signal provided by a pressure sensor situated upstream in front of the particulate filter, and the pressure downstream of the particulate filter which is determined with the aid of a pressure model of the exhaust-gas system existing downstream of the particulate filter, in which model the ambient-air pressure at the end of the exhaust-gas system is taken into account.  
      A particulate filter is regenerated by a burn-off of the particulates embedded in the particulate filter, which takes place in a temperature range of 500° C. -650° C., for example. German Patent Application No. DE 101 08 720 describes a method and a device for operating a particulate filter situated in an exhaust-gas region of an internal combustion engine, in which the starting point is at least one operating characteristic that indicates the state of the internal combustion engine and/or the state of the particulate filter, and from which a characteristic quantity is determined that describes the intensity of the particulate burn-off. The characteristic quantity is compared to a threshold value. If the threshold value is exceeded, measures are initiated for reducing the reaction speed in order to prevent overheating of the particulate filter, the measures being directed toward interventions to reduce the oxygen content in the exhaust gas.  
      German Patent Application No. DE 196 02 599 describes a method for determining a quantity of motor oil in an internal combustion engine, in which the oil level is measured by an oil sensor. The method makes it possible to ascertain the oil level comparatively accurately during operation of a motor vehicle.  
      An object of the present invention is to provide a method for operating a particulate filter disposed in an exhaust-gas region of an internal combustion engine and a device for implementing the method which permit the most precise possible ascertainment of the particulate load state of the particulate filter.  
     SUMMARY OF THE INVENTION  
      The procedure of the present invention provides that the oil level of the internal combustion engine is sensed by an oil sensor, that an oil-consumption determination unit determines the oil consumption of the internal combustion engine based on the oil signal provided by the oil sensor, and that an ash-load determination unit determines a measure for the ash-load state of the particulate filter from the oil consumption.  
      In addition to the particulates formed due to the combustion of the fuel in the cylinders of the internal combustion engine, ashes become embedded which result from the burning of motor oil, especially motor-oil additives. The ash accumulating in the particulate filter cannot be removed from the embedded particulates within the framework of regenerating the particulate filter. The knowledge of the ash load state may advantageously be taken into consideration during operation of the particulate filter.  
      One embodiment provides that a particulate-load determination unit determines the particulate-load state of the particulate filter, and that the ash-load state is taken into consideration when determining the particulate-load state. This measure is particularly advantageous if the particulate-load state is determined from the differential pressure occurring at the particulate filter. For example, the influence of the ash-load state on the differential pressure may be determined experimentally and utilized later for correction of the particulate-load state.  
      A further refinement of this embodiment provides that the exhaust-gas pressure upstream of the particulate filter is sensed by a pressure sensor, and that the exhaust-gas pressure downstream of the particulate filter is ascertained with the aid of a pressure model of the exhaust-gas system existing downstream of the particulate filter, in which model the ambient-air pressure occurring at the end of the exhaust-gas system is taken into account.  
      Another development provides that a measure for the temperature in the particulate filter is taken into account when ascertaining the particulate-load state. For example, the temperature may be calculated with the aid of an exhaust-gas temperature model. The temperature may moreover be sensed by at least one temperature sensor situated in the region of the particulate filter. The temperature sensor may be arranged in front of and/or in and/or after the particulate filter.  
      One particularly advantageous refinement provides that a threshold value predefined for the maximum permissible particulate-load state of the particulate filter or a predefined tolerance range is stipulated as a function of the ascertained ash-load state. This refinement takes into account that for safety reasons, the maximum permissible particulate-load state must be reduced as the ash-load state increases. Depending on the mechanical form of the particulate filter, with increasing ash-load state, the layer thickness of the embedded particulates increases given the same particulate-load state. During the necessary regeneration by burn-off of the particulates, with increasing thickness of the particulate layer, the danger of overheating may develop which can be decreased by reducing the maximum permissible particulate-load state.  
      Another further development of the procedure according to the present invention provides that, from the ascertained ash-load state, a particulate-filter service-life determination unit, using a filter-replacement signal, indicates a necessary replacement of the particulate filter.  
      The device according to the present invention for implementing the method according to the present invention relates first to a control device that is adapted for implementing the method.  
      In particular, the control device includes an oil sensor which provides an oil signal that is at least one measure for the oil level.  
      The device of the present invention further provides that a pressure sensor arranged upstream of the particulate filter is embodied as a differential-pressure sensor which ascertains the pressure difference between the exhaust-gas pressure upstream of the particulate filter and the ambient-air pressure.  
      The control device preferably includes at least one electrical memory in which the functions are stored as a computer program. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a technical environment in which a method of the present invention proceeds.  
       FIG. 2  shows a functional correlation between an ash-load state and a differential pressure.  
       FIG. 3  shows functional correlations between the particulate-load state and a differential pressure. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows an internal combustion engine  10  that has an air sensor  11  and a throttle valve  12  situated in its intake region. In the exhaust-gas region of internal combustion engine  10 , a temperature sensor  21  and a pressure sensor  22  are provided upstream in front of a particulate filter  20 .  
      A further exhaust-gas treatment device  23  is connected downstream of particulate filter  20 .  
      A fuel-metering device  25  is assigned to internal combustion engine  10 . Oil level  26  in internal combustion engine  10  is detected by an oil sensor  27 .  
      A control device  30  receives an air signal ml from air sensor  11 , a speed signal N from internal combustion engine  10 , an oil signal H from the oil sensor, an exhaust-gas temperature T from the temperature sensor and a pressure signal p from pressure sensor  22 . Control device  30  provides a throttle-valve signal DR to throttle valve  12 , and a fuel signal mE to fuel-metering device  25 .  
      Pressure sensor  22  provides pressure signal p as a function of exhaust-gas pressure pvPF of exhaust-gas mass flow msabg upstream of particulate filter  20  and as a function of ambient-air pressure pU. Downstream of particulate filter  20 , exhaust-gas pressure pnPF of exhaust-gas mass flow msabg occurs after particulate filter  20 . Ambient-air pressure pU occurs at the end of the exhaust-gas region.  
      Control device  30  includes an oil-consumption determination unit  31  that emits an oil-consumption signal  32  to an ash-load determination unit  33 . Ash-load determination  33  provides an ash-load state mAsh to a particulate-load determination unit  34 , a threshold establishment unit  35  and a particulate-filter service-life determination unit  41 . The particulate-filter service-life determination provides a filter-replacement signal  42 .  
      Particulate-load determination  34  determines particulate-load state mParticulate as a function of a differential pressure dp and as a function of exhaust-gas temperature T. Differential pressure dp is provided by a differential-pressure determination unit  36  as a function of pressure signal p and ambient-air pressure pU which is detected by an ambient-air-pressure sensor  37 . Particulate-load state mParticulate and a threshold value Lim are fed to a regeneration coordinator  38  that emits a regeneration signal  39  to a regeneration control  40 .  
       FIG. 2  shows a functional correlation between ash-load state mAsh and differential pressure dp which holds true given a constant exhaust-gas volumetric flow Vs.  
       FIG. 3  shows functional correlations between particulate-load state mParticulate and differential pressure dp. Ash-load state mAsh is indicated in unit percentage. A first characteristic curve  50  holds true for an ash-load state mAsh of zero %, a second characteristic curve  51  for an ash-load state mAsh of 20%, and a third characteristic curve  52  for an ash-load state mAsh of 50%. Threshold value Lim is plotted.  
      The method of the present invention operates as follows:  
      Control device  30  initially establishes fuel signal mE, fed to fuel-metering device  25 , as well as throttle-valve signal DR as a function of air signal ml and/or speed signal N and/or torque setpoint signal MFa. During normal operation of internal combustion engine  10 , exhaust-gas mass flow msabg carries along particulates that are formed during the combustion process of the fuel in internal combustion engine  10 , especially upon combustion of fuel additives. The particulates become embedded in particulate filter  20 .  
      Further exhaust-gas treatment device  23  situated downstream of particulate filter  20  is a catalytic converter or a muffler, for example.  
      The motor oil necessary for operating internal combustion engine  10  is monitored by oil sensor  27 , at least with respect to fluid level  26 . Oil sensor  27  emits oil signal H to control device  30 , in which oil-consumption determination  31  determines oil-consumption signal  32 .  
      The oil consumption is determined based on the decrease of fluid level  26 . An increase of fluid level  26  due to a replenishment of consumed motor oil must be taken into account when evaluating a change in fluid level  26 .  
      Ash-load determination  33  assigns ash-load state mAsh of particulate filter  20  to oil-consumption signal  32  with the aid of a correlation stored in ash-load determination  33 . The correlation, not shown in more detail, is preferably determined experimentally.  
      The knowledge of ash-load state mAsh may be taken into consideration particularly advantageously during operation of particulate filter  20 .  
      The increasing ash-load state impairs the storage capability and the working manner of particulate filter  20 . Since it is not readily possible to remove the ash when particulate filter  20  is in the installed state, according to one refinement, ash-load state mAsh may be used to signal that it is necessary to replace particulate filter  20 . After reaching a predefined ash-load state mAsh, particulate-filter service-life determination  41  outputs filter replacement signal  42 .  
      Ash-load state mAsh may moreover be taken into account especially advantageously in the determination of particulate-load state mParticulate of particulate filter  20 . In the exemplary embodiment, particulate-load determination  34  determines particulate-load state mParticulate as a function of differential pressure dp which occurs at particulate filter  20 , and as a function of exhaust-gas temperature T. Ash-load state mAsh has an influence on differential pressure dp, which is shown in  FIG. 2 . The correlation is a function of the form of particulate filter  20 . With the aid of experimentally verified calculations it was found that the case can occur that, starting from a low ash-load state mAsh, with increasing ash-load state mAsh, contrary to expectation, differential pressure dp initially assumes smaller values, and only with further increasing ash-load state mAsh does it rise to the anticipated higher values. One measure for particulate-load state mParticulate of particulate filter  20  is the flow resistance of particulate filter  20  which, according to the related art indicated at the outset, results from the quotient of differential pressure dp and exhaust-gas volumetric flow Vs.  
      Exhaust-gas volumetric flow Vs is calculated from exhaust-gas mass flow msabg, taking into account exhaust-gas temperature T that is detected by temperature sensor  21 . In simple approximation, exhaust-gas mass flow msabg is proportional to air signal ml. To increase the accuracy, the fuel burned in internal combustion engine  10  may be taken into account with inclusion of fuel signal mE. If an exhaust-gas recirculation is present, the influence on exhaust-gas mass flow msabg may likewise be included.  
      The three characteristic curves  50 ,  51 ,  52  shown in  FIG. 3  indicate the correlation between differential pressure dp and particulate-load state mParticulate in percentage. The correlation, shown in  FIG. 2 , between ash-load state mAsh and differential pressure dp is expressed in  FIG. 3  in that, given low particulate-load state mParticulate, second characteristic curve  51 , which corresponds to an ash-load state mAsh of 20%, is at a lower differential pressure dp than in the case of a lower ash-load state mAsh, which is plotted in  FIG. 3  with first characteristic curve  50 , corresponding to an ash-load state of zero %. According to  FIG. 2 , differential pressure dp first increases again with rising ash-load state mAsh. Therefore, third characteristic curve  52 —as expected—lies above first characteristic curve  50 , that is to say, the increased ash-load state, indicated with 50% in the example, given the same particulate-load state mParticulate, leads—as expected—to a higher differential pressure dp.  
      Predefined threshold value Lim is entered in  FIG. 3 . If, according to first characteristic curve  50 , no ash is yet embedded in particulate filter  20 , threshold value Lim may be set to the maximum permissible particulate-load state mparticulate of 100%. With increasing ash-load state mAsh, threshold value Lim for permissible particulate-load state mParticulate is reduced. In the exemplary embodiment shown according to  FIG. 3 , given an ash-load state mAsh of 50% according to third characteristic curve  52 , threshold value Lim is reduced to, for example, 50% permissible particulate-load state mParticulate, as well. The correlation between the reduction of permissible particulate-load state mParticulate as a function of existing ash-load state mAsh may be ascertained theoretically or with the aid of experiments. The goal is to ensure a burn-off of the particulates during the regeneration, without the danger of overheating particulate filter  20 . The reduction of permissible particulate-load state mParticulate in relation to existing ash-load state mAsh may deviate considerably from the pattern shown in  FIG. 3 . For example, given an ash-load state of 50% according to third characteristic curve  52 , threshold value Lim could be at only 20% of maximum possible particulate-load state mParticulate.  
      Threshold value Lim set in threshold establishment  35  as a function of ash-load state mAsh, as well as particulate-load state mParticulate determined in particulate-load determination  34  are fed to regeneration coordinator  38  which ascertains whether a regeneration of particulate filter  20  is necessary. If this is the case, regeneration coordinator  38  emits regeneration signal  39  to regeneration control  40 , which takes suitable measures for regenerating particulate filter  20 .  
      For example, regeneration control  40  influences fuel signal mE which is supplied to fuel-metering device  25 . For instance, fuel signal mE causes fuel-metering device  25  to release at least one secondary injection that is supposed to occur after a main injection. The post-injected fuel quantity burns—if at all—only in part in the cylinders of internal combustion engine  10 . In each case, unburned fuel arrives in an optionally provided oxidation catalytic converter (not shown more precisely) situated upstream in front of particulate filter  20 . The unburned fuel is converted in an exothermic reaction in the oxidation catalytic converter and contributes to the rise in the exhaust-gas temperature. Particulate filter  20  may itself have a catalytically acting coating on which the exothermic reaction takes place, so that direct heating of particulate filter  20  is possible.  
      Moreover, as a function of air signal ml, regeneration control  40  may influence throttle-valve signal DR for throttle valve  12  in such a way that the air flow is throttled, which leads to a further elevation of exhaust-gas temperature T. The ignition temperature of the particulates embedded in particulate filter  20  is reached by the elevation of the exhaust-gas temperature to 500-650° C., for example. The burn-off speed may be influenced by controlled influencing of the oxygen content in the exhaust gas. Regeneration control  40  adjusts the oxygen content by influencing throttle-valve signal DR.  
      Differential-pressure determination  36  calculates differential pressure dp, occurring at particulate filter  20 , from pressure signal p provided by pressure sensor  22 , from ambient-air pressure pU measured by ambient-air-pressure sensor  37 , and with reference to a pressure model of the exhaust-gas system downstream of particulate filter  20 . The exhaust-gas system downstream of particulate filter  20  includes further exhaust-gas treatment device  23  which is embodied at least as a catalytic converter and/or at least as a muffler, for instance. In forming the model, for example, the flow resistance of further exhaust-gas treatment device  23  and the flow resistance of the exhaust pipes are calculated as a function of exhaust-gas volumetric flow Vs, which—as already described—may be determined from exhaust-gas mass flow msabg with inclusion of exhaust-gas temperature T. Exhaust-gas pressure pnPF downstream of particulate filter  20 , which is the goal of the calculation with the aid of the pressure model, may be determined from the flow resistance, exhaust-gas volumetric flow Vs, as well as known ambient-air pressure pU.  
      Ambient-air-pressure sensor  37  is disposed within control device  30 , for example, since the ambient-air pressure may be utilized in particular for influencing fuel signal mE. Pressure sensor  22  is preferably in the form of a differential-pressure sensor which measures exhaust-gas pressure pvPF upstream of particulate filter  20  in comparison to ambient-air pressure pU and provides the pressure difference as pressure signal p.