Abstract:
Proposed is a method for assessing the completeness of a regeneration of a particle filter which is traversed by the exhaust gas of an internal combustion engine, wherein a pressure difference which is generated across the particle filter when a flow passes through the particle filter is measured and evaluated for the assessment. The method is characterized in that a time derivative of the measured pressure difference is formed and the assessment of the completeness of the regeneration takes place as a function of the time derivative.

Description:
TECHNICAL FIELD 
       [0001]    The invention concerns a procedure according to the generic term of claim  1 , the application of such a procedure, as well as a control unit according to the generic term of claim  6 . 
       BACKGROUND 
       [0002]    Such a procedure, such an application and such a control unit are already known from the commercial use at motor vehicles with diesel engines. Particle filters present effective measures for reducing the emissions of soot at combustion engines, in particular diesel engines. The soot particles that are contained in the exhaust gas deposit in the particle filter when passing through the particle filter. With an increasing soot load of the particle filter the exhaust gas pressure increases. As a result the efficiency of the combustion engine declines. The fuel consumption increases and the motor vehicle accelerates worse. In order to limit the negative influence of the soot load, the particle filter has to be freed from time to time from the stored root particles, which is also called regeneration. The regeneration takes place by combusting the soot that is stored in the particle filter with oxygen from the exhaust gas into carbon dioxide. 
         [0003]    The combustion is actuated by power-operated measures, which cause an increase of the exhaust gas temperature. This is achieved by interventions into the air system of the combustion engine, for example a throttling, and/or interventions into the injection process. The regeneration begins when the exhaust gas temperature exceeds the ignition temperature of the soot in the particle filter. 
         [0004]    At a motor vehicle such a regeneration typically takes place depending on the load of the soot particle filter with air after a driving distance of approximately 300 to 800 km. the load depends on the soot raw emissions of the combustion engine and the size of the particle filter. For the detection of the load the signal of a pressure difference sensor is evaluated at the known subject matter, which detects a pressure difference, which generates at the pass through the particle filter. Under the condition of a constant exhaust gas volume flow the pressure difference increases with an increasing load of the particle filter with soot. The pressure difference that standardized to the volume flow provides a dimension for the flow resistance and therefore for the load of the particle filter with soot. The control unit calculates such a dimension depending on operating parameters of the combustion engine and controls the regeneration of the particle filter depending on the mentioned dimension. 
         [0005]    A regeneration is for example initiated when the standardized pressure difference exceeds a first threshold. An initiated regeneration usually lasts several minutes and ends at the known subject matter when the quotient of the pressure difference and the exhaust gas volume flow falls below a lower threshold. 
       SUMMARY 
       [0006]    For a correct operation of the exhaust gas purification system with the particle filer it is important that the regeneration takes place at the right point of time and is implemented as complete as possible. Repeatedly occurring incomplete regenerations are disadvantageous for the following reasons: the good efficiency of modern combustion engines, in particular modern diesel engines, is accompanied with correspondingly low exhaust gas temperatures and particle filter temperatures. Each regeneration causes a temperature variation in stress due to the high exhaust gas temperature that is necessary for the soot combustion, which lets the particle filter age. A high frequency of initiated regenerations causes therefore a faster ageing and therefore an undesired shortening of the operational life span of the particle filter. Besides each regeneration requires a certain fuel expenditure for heating the exhaust gas system. Incomplete regenerations increase the fuel consumption and further, because they cause an increased exhaust gas pressure in a timely average, which has a disadvantageous effect on the efficiency of the combustion engine. 
         [0007]    Surprisingly it showed that the known evaluation of the quotient of pressure difference and exhaust gas mass flow at the operation of a diesel motor with a mostly constant exhaust gas mass flow, thus especially at an operation with an exhaust gas mass flow, which is mostly constant during and after the regeneration, is not sufficient to detect an incomplete regeneration of the particle filter reliably. 
         [0008]    Regenerations controlled by the known procedure are especially terminated often too early at these combustion engines. That causes that the particle filter is correspondingly fast loaded again, so that a new regeneration attempt is initiated. Summed up, this results in an inadequate high frequency of regeneration attempts, which causes the mentioned disadvantages of a hastily ageing and an increased fuel consumption. 
         [0009]    It is also disadvantageous that familiar control functions for controlling the completeness of the regeneration are based on an evaluation of a frequency of initiated regeneration attempts. Therefore a comparably long time lapses at the state of the art, in which the combustion engine is operated with an averagely too high exhaust gas pressure and therefore with an increased fuel consumption, until determining a not sufficient regeneration, which causes a high value of the frequency. 
         [0010]    Based on this the task of the invention is the representation of an improved detection of an incomplete regeneration of a particle filter, whereby the representation concerns procedure aspects as well as application aspects and device aspects. 
         [0011]    This task is each solved by the characteristics of the independent claims. It has shown that the evaluation of the time derivative of the detected pressure difference according to the invention allows more reliable statements about the completeness of a regeneration than the familiar evaluation of the pressure difference. A further advantage is that the invention allows an evaluation of the regeneration success already directly after an individual regeneration of the particle filter. Therefore even system errors can be detected faster than at the known procedure, which evaluates a frequency of regeneration attempts and therefore has to wait for several regeneration attempts. By a faster detection of incomplete regenerations it can be intervened faster into the control of the regeneration. The increases of the fuel consumption and the age based wearout of the particle filter that are associated with the too frequently initiated regenerations can therefore be prevented or at least reduced. 
         [0012]    Further advantages arise from the dependent claims, the description and the attached figures. 
         [0013]    It is self-evident that the previously mentioned and subsequently explained characteristics can not only be applied in the stated combination, but also in other combinations or alone, without leaving the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Embodiments of the invention are illustrated in the drawings and further explained in the subsequent description. It is schematically shown: 
           [0015]      FIG. 1  is a combustion engine, which provides an exhaust gas system with a particle filter; 
           [0016]      FIG. 2  is an embodiment of a procedure according to the invention in the form of a flow diagram; 
           [0017]      FIG. 3  is a time course of the difference pressure over the particle filter after a complete and after an incomplete regeneration, and 
           [0018]      FIG. 4  shows values of the time derivative of the difference pressure in an application over the regeneration success. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows among others a cross-section of a particle filter  10 , in which exhaust gas of a combustion engine  12  flows from left to right. The particle filter  10  provides a porous filter structure  14 , which is permeable for the exhaust gas, but does not let through soot particle that are contained in the exhaust gas or only a small part. The porous filter structure  14  provides in the embodiment in  FIG. 1  either way left or right closed channels  16 ,  18 , which are separated from each other by porous walls  20 . In order to pass the particle filter  10 , the exhaust gas has to flow through the porous walls  20 . Soot particles that are in the exhaust gas deposit thereby in the pores of the walls  20 , which blocks these gradually and increases the flow resistance of the particle filter  10 . 
         [0020]    The flow resistance causes a pressure drop at the through-flow of the particle filter  10 , which is detected as a difference pressure dp by a difference pressure sensor  22  and transferred to a control unit  24 . In the following the difference pressure as well as the pressure signal are termed with the same reference sign dp. The control unit  24  creates a dimension for the load of the particle filter  10  with soot depending on the pressure signal dp. Depending on the arrangement the pressure signal dp can also be used as a dimension for the load, since the pressure difference dp over the particle filter  10  grows monotonously with the load of the particle filter  10 . 
         [0021]    The control unit  24  is preferably a control unit, which creates correcting variables S_L and S_K for the operation of the combustion engine  12  depending on the signal dp of the difference pressure sensor  22  and signals BP, T of further sensors  26 ,  28 . Operating parameters of the combustion engine  12 , such as the engine speed, intake air mass, load pressure, driver request, combustion chamber pressure, combustion noises and so on, show in the signals BP of sensors  26 , whereby this list is not meant to be final and the control unit  24  also does not have to proceed signals to all mentioned operating parameters. The sensor  28  detects a temperature T before or in the particle filter  10 . In a preferred embodiment this temperature T serves as actual value for a temperature regulation, with which the particle filter temperature is kept above the ignition temperature of the soot during a regeneration of the particle filter  10 . With the corrective variables S_L the control unit  24  intervenes into an air system  30  (there for example into a throttle valve or an exhaust gas recirculation valve) and with the corrective variable S_K into a fuel system  32  (there for example into an injector arrangement) of the combustion engine  12 . 
         [0022]    Besides the control unit  24  is customized, in particular programmed, to control a course of the procedure according to the invention or a course of one or several embodiments. 
         [0023]      FIG. 2  shows an embodiment of the procedure according to the invention in the form of a flow diagram. Step  34  thereby represents a main program HP for controlling the combustion engine  12 , which is processed in the control unit  24 . In the main program of step  34  the combustion engine  12  is operated in a normal operation NB. The term normal operation serves to distinguish it from the term regeneration operation and comprises therefore all operation types BA, in which no regeneration of the particle filter  10  takes place. Therefore the selected operation type BA is the normal operation NB (BA=NB) in the main program of step  34 . 
         [0024]    For checking the load of the particle filter  10  with soot the main program from step  34  branches repeatedly into a step  36 , in which the difference pressure dp is detected over the particle filter  10 , as it is provides by the difference pressure sensor  22 . A step  38  follows that by comparing the value of the detected pressure difference dp with a threshold value SW 1 . As long as dp does not exceed the threshold value SW 1 , the program branches back into step  34 , in which the combustion engine  12  is continued to be operated in normal operation NB. 
         [0025]    If the difference pressure dp however exceeds the threshold value SW 1 , step  40  follows step  38 , in which the operation type BA of the combustion engine  12  is switched from or reversed to normal operation NB into a regeneration operation RB. In this regeneration operation RB the control unit  24  controls the combustion engine  12 , so that the temperature of its exhaust gases is increased so high that the ignition temperature of the soot in the particle filter  10  is reached or exceeded. This takes place preferably by changing the corrective variables S_L for the air system  32  and/or S_K for the fuel system  30 . 
         [0026]    For increasing the exhaust gas temperature early, burning or accumulated after injections, a late time shift of the main injection and an inlet air throttling or an increase of the exhaust gas recirculation rate comes into question. Combinations of this measure are also possible. Alternatively or additionally also late after injections come into question, which cause a further exhaust gas temperature increase by oxidizing the fuel that is not burned anymore in the combustion chamber in an oxidation catalyzer. 
         [0027]    In order to provide a reliable regeneration of the particle filter  10  even at disadvantageous environment conditions, the exhaust gas temperature is regulated at the regeneration in a preferred embodiment. For the regulation of the exhaust gas temperature the temperature signal of the temperature sensor  28  serves as an initial value, which is arranged before or in the particle filter  10 . Alternatively or additionally to the detection of an actual temperature of the particle filter or the exhaust gas such an actual temperature can also be calculated in the control unit  24  from operating parameters of the combustion engine  12  like the sucked in air amount and injected fuel amount by an exhaust gas temperature model that is realized as a software module. 
         [0028]    Subsequent to step  40  the difference pressure dp that is present at the regeneration operation RB over the particle filter  10  is detected in step  42  and compared in step  44  with a second threshold value SW 2 . The second threshold value SW 2  is smaller than the first threshold value SW 1 . It is fallen below, if the particle filter  10  is almost completely regenerated. T the beginning of the regeneration the difference pressure dp is nevertheless not higher than the threshold value SW 2 , so that the request in step  44  is negated and the program branches back to step  40 , in which the regeneration operation RB is continued. The regeneration operation RB is continued by a repeated pass through the loop from steps  40 ,  42  and  44  so long until it is determined in step  44  that the difference pressure dp falls below the second threshold value SW 2  due to an advancing regeneration of the particle filter  10 . 
         [0029]    In that case the mentioned loop from steps  40 ,  42  and  44  is left and the program branches into step  46 , in which the operation type BA of the combustion engine  12  is controlled back from the regeneration operation RB into normal operation NB. Thereby a time or count variable t of an internal timer of the control unit  24  is set to an initial value t 0 . Subsequently in step  48  a new detection of the difference pressure dp takes place. In step  50  the time derivative z=d/dt (dp) is created. In step  52  the time derivative z and therefore the changing speed of the difference pressure dp is compared with a threshold value z_S. 
         [0030]    At an almost complete regeneration of the particle filter  10  the difference pressure dp increases only slowly after the regeneration. The increase is correspondingly flat and the value of the time derivative z correspondingly small, so that the threshold value z_S is not exceeded in step  52 . In that case steps  54  follows, in which the time or count variable t is compared with a threshold value t_S. This will not yet be the case, so that the request is negated in step  54  and step  56  follows, in which the value of the time or count variables t is increased by an increment dt. 
         [0031]    This is followed by a new detection of the difference pressure dp in step  48 . The loop from steps  48 ,  50 ,  52 ,  54  and  56  is then repeatedly passed through until the value of the time or count variable t exceeds the threshold value t_S in step  54  by a repeated incrementing. The program branches then into step  58 , in which the regeneration R is assessed as complete. Altogether a time derivative z that is created after the end of the regeneration is compared with a threshold z_S and the regeneration then assessed as successful, if the time derivative z is smaller than a default threshold. Subsequently the procedure returns into the main program of step  34 . The time request in step  54  causes that the value z of the increase is controlled no more than for the duration of a time span t_S. the comparison with the threshold for the increase takes therefore only place within a defined time span t_S after the end of the regeneration. 
         [0032]    If the regeneration R has not been sufficient, the difference pressure dp initially increases comparably fast in the subsequent normal operation NB. Thereby a comparably high value of the derivative z results in step  50 , so that the threshold value z_S is exceeded in step  52  at a sufficiently incomplete regeneration R. the procedure branches then from step  52  into step  60 , in which the regeneration R is assessed as incomplete. In contrast to the state of the art this assessment takes place right after a single regeneration. 
         [0033]      FIG. 3  shows a course of the difference pressure dp over the time t. At the point of time t_x the particle filter is completely unloaded of soot particles and is then subsequently loaded over a period of time in the dimension of hours. The tangent  64  clarifies thereby the beginning increase of the dp(t)-curve, which decreases in the further course. At the point of time t  1  the particle filter is regenerated. In contrast to the point of time t_x the regeneration is only incomplete at the point of time t_y. thus only 10% of the accumulated soot particles are burned t the point of time t_y. Like the strong drop-down in the pressure course shows, the difference pressure dp sinks thereby already to approximately half the value that it had before the regeneration, thus shortly before the point of time t_y. Due to this clear reaction of the pressure difference one would expect an extensive regeneration success. The fast advance of the pressure difference at the new loading of the particle filter, which takes place after the point of time t_y, is typical for an incomplete regeneration. The fast advance can also be noted in the advance of the tangent  66 . From a comparison of the tangent  66  and  64  it is qualitatively apparent that the incomplete regeneration at the point of time t_y comes along with a higher value of the tangent advance than the more complete regeneration at the point of time t_x. 
         [0034]      FIG. 4  clarifies this relation more. Curve  68  shows the relation of initial ascents of the difference pressure dp, thus the value of its time derivative d/dt(dp) shortly after a regeneration over the values of the regeneration success in %, which are illustrated along the x-coordinate. The percentage provides thereby the ratio of burned soot particles compared to accumulated soot particles. The value of 100% corresponds thereby a complete regeneration. At a load with 10 g soot and a combustion of 4 g soot at a regeneration this results for example in a value of 40% for the regeneration success. 
         [0035]    Regeneration successes, which are higher than 10%, are shown reproducibly in a monotonously falling course of the curve  68  and allow therefore a quantitative evaluation of the regeneration success already if only one regeneration took place. In particular it can be clearly seen that the time derivative d/dt(dp) has significantly higher values in the case of severe incomplete regenerations than in the case of a complete regeneration. 
         [0036]    It has shown that this procedure also allows reliable assessments of the regeneration success at an operation of diesel engines in stationary operation points. A preferred embodiment provides therefore an application of the method or one of its embodiments at a combustion engine, in particular a diesel engine, which is operated in stationary operating points. But the application is not limited to applications with only stationary operating points. Thus a further embodiment provides, that a dimension for the exhaust gas mass flow of the combustion engine is created in non-stationary operated combustion engines and that only such results are used, which have been won at a mostly constant exhaust gas mass flow.