Abstract:
invention relates to a method for operating an internal combustion engine, in the exhaust gas section of which is arranged at least one SCR catalytic converter, which is hit with a reagent that contributes to NO x  conversion in the SCR catalytic converter, and to a device for implementing the method. At least one measure is calculated for the NO x  concentration downstream after the SCR catalytic converter. An NO x  sensor, which is arranged downstream after the SCR catalytic converter, generates an exhaust gas sensor signal which corresponds to at least the NO x  concentration and optionally, as a result of cross sensitivity, to a reagent slip. A reagent signal, which determines the dosage of reagent, is influenced by a correction signal as a function of the difference and as a function of a measure for the temperature of the SCR catalytic converter.

Description:
TECHNICAL FIELD 
     The invention proceeds from a procedure to operate an internal combustion engine, in whose exhaust gas section, a NO x  sensor is disposed downstream after the SCR catalytic converter; and from a device to implement the procedure according to the class of the independent claims. 
     BACKGROUND 
     In the German patent DE 199 03 439 A1 a procedure and a device to operate an internal combustion engine are described, in whose exhaust gas section a SCR catalytic converter (Selective Catalytic Reduction) is disposed, which reduces the nitrogen oxides contained in the exhaust gas of the internal combustion engine to nitrogen using a reagent substance. The metering of the reagent substance is carried out preferably as a function of the operating parameters of the internal combustion engine, such as, for example, the engine rotational speed and the amount of fuel injected. Furthermore, the metering is carried out preferably as a function of the exhaust gas parameters, such as, for example, the exhaust gas temperature or the operating temperature of the SCR catalytic converter. 
     Provision is made, for example, for ammonia to be the reducing agent, which is derived from a urea water solution. The dosage of the reagent substance or of the source materials of the reagent substance must be carefully established. A dosage which is too small has the consequence that the nitrogen oxides in the SCR catalytic converter can no longer be completely reduced. Too large of a dosage leads to a slip of the reagent substance, which on the one hand can lead to an unnecessarily large consumption of the reagent substance, and on the other hand as a function of the composition of the reagent substance can lead to an unpleasant odor. 
     In the German patents DE 199 60 731 A1 and DE 199 62 912 A1, NO x , sensors are described in each case, in which provision is made for the acquisition of the NO x  concentration present in an exhaust gas flow. The NO x  sensors contain multiple chambers, which are connected to each other via diffusion barriers. The known multiple chamber NO x  sensors have as a result of the measuring principle a cross sensitivity with regard to ammonia (NH3). For example as a reagent substance, the ammonia contained in the exhaust gas leads to a falsification of the sensor signal by way of the reactions 4 NHS+5O2→4NO+6H2O. If an increase of the reagent substance dosage thus occurs during the previously known procedural approaches, the sensor signal will increase when an excess dosage or a correct dosage of the reagent substance exists due to the reagent substance slip which arises; and when an underdosage of the reagent substance exists due to the increasing NO x  conversion, the sensor signal will drop out. If on the other hand the reagent substance dosage is lowered, the sensor signal will drop out if an excess dosage of the reagent substance exists due to the decreased reagent substance slip; and the sensor signal will increase when a correct dosage or an underdosage of the reagent substance exists due to the NO x  conversion, which is no longer complete. 
     In the German patent DE 10 2004 046 640 A1, a procedure to operate an internal combustion engine and a device to implement the procedure are described, in which a NO x  sensor with a cross sensitivity with regard to a reagent substance is disposed downstream after the SCR catalytic converter. At least one SCR catalytic converter, which is impinged with a reagent substance, is disposed in the exhaust gas section. The reagent substance contributes to the NO x  conversion in the SCR catalytic converter. Provision is made for the calculation of at least one measurement for the NO x  concentration arising downstream after the SCR catalytic converter. This calculation makes an increase in accuracy when establishing a dosage of the reagent substance possible. 
     A reagent substance slip can be ascertained from the difference between the calculated measurement for the NO x  concentration and the measured quantity for the sum of the NO x  concentration and the reagent substance concentration. The fact is taken into account that a reagent substance slip as well as an insufficient NO x  reducing reaction causes a deviation in the same direction between the calculated measurement for the NO x  concentration and the measured quantity for the sum of the NO x  concentration and the reagent substance concentration. According to one embodiment the dosage of the reagent substance is initially reduced when a difference as mentioned above occurs. If a reagent substance slip were present, the reduction of the dosage of the reagent substance would lead to a reduction of the reagent substance slip. The reduction of the dosage of the reagent substance proved in this case to be the correct step. If too small a dosage of the reagent substance were originally present, the difference ascertained would continue to increase due to a small NO x , conversion, so that it could be suggested from this, that the reduction of the reagent substance was false and an increase of the dosage is to be conducted instead. 
     In the German patent DE 10 2004 031 624 A1, a procedure is described to operate a SCR catalytic converter used to purify the exhaust gas of an internal combustion engine, in which provision is made for an open-loop or closed-loop adjustment of the reagent substance fill level in the SCR catalytic converter to a specified storage set point. The targeted specification of the storage set point secures on the one hand that during transient states of the internal combustion engine, a sufficient amount of the reagent substance is available to remove the NO x  emissions of the internal combustion engine before the SCR catalytic converter as completely as possible; and that on the other hand, a reagent substance slip is avoided. 
     The reagent substance fill level of the SCR catalytic converter is ascertained using a catalytic converter model, which takes into account the NO x  mass flow entering into the SCR catalytic converter, the NO x  mass flow departing the SCR catalytic converter, the temperature of the catalytic converter as well as if need be the reagent substance slip. The maximum possible reagent substance fill level of the SCR catalytic converter depends especially on the operating temperature of the SCR catalytic converter, which is the highest at low operating temperatures and drops off to lower values with an increasing operating temperature. The efficiency of the SCR catalytic converter depends on the catalytic activity; which is small at low operating temperatures, passes through a maximum with a rising operating temperature and drops off again when the operating temperature continues to rise. 
     The task underlying the invention is to indicate a procedure to operate an internal combustion engine, in whose exhaust gas section a SCR catalytic converter and a NO x  sensor located downstream after the SCR catalytic converter are disposed, as well as a device to implement the procedure, which allows for the most optimal as possible result for the purification of the exhaust gas simultaneously with a minimal reagent substance slip. 
     The task is solved in each case by the characteristics indicated in the independent claims. 
     SUMMARY OF THE INVENTION 
     The procedure according to the invention to operate an internal combustion engine assumes that at least one SCR catalytic converter is disposed in the exhaust section of the internal combustion engine. The SCR catalytic converter is impinged with a reagent substance, which contributes to the NO x  conversion in the SCR catalytic converter. Downstream after the catalytic converter, a NO x  catalytic converter is disposed, which provides a sensor signal, which reflects at least a measurement for the NO x  concentration in the exhaust gas downstream after the catalytic converter. At least one measurement is additionally calculated for the NO x  concentration arising downstream after the SCR catalytic converter. The difference is ascertained between the calculated measurement for the NO x  concentration and the measured NO x  concentration. A reagent substance signal, which establishes the dosage of the reagent substance, is not only affected by a correction signal as a function of the ascertained difference, but additionally as a function of a measurement for the temperature of the SCR catalytic converter. 
     When establishing the dosage of the reagent substance, the procedural approach according to the invention takes into account the temperature of the SCR catalytic converter, which has an affect on the storage capability of the reagent substance in the SCR catalytic converter. A change of the reagent substance signal, which is required if necessary, is thereby adapted to the temperature dependent storage capability of the reagent substance in the catalytic converter. With this step, an excess dosage of the reagent substance, which arises contingent on a change of the reagent substance and would lead to an increased reagent substance slip, as well as an underdosage of the reagent substance, which would lead to and increased NO x  impact on the environment, is avoided in each case. 
     The procedural approach according to the invention can be implemented with simple and cost effective means so that merely a NO x  sensor disposed downstream after the SCR catalytic converter is required, which if need be has an inherently undesirable cross sensitivity with regard to the reagent substance. This cross sensitivity is specifically utilized within the scope of an embodiment of the procedural approach according to the invention. 
     Advantageous modifications and embodiments of the procedural approach according to the invention result from the dependent claims. 
     Provision is made in one embodiment for the correction signals to be deposited as a function of the difference and of the measurement for the temperature of the SCR catalytic converter in a characteristic diagram, which supplies a selected correction signal. 
     Provision is made in one embodiment for the reagent substance signal to be established as a function of the reagent substance fill level in the SCR catalytic converter. Provision is made in a modification of this embodiment for the reagent substance signal to be indirectly affected by way of a manipulation of the reagent substance fill level in the SCR catalytic converter. Preferably the reagent substance fill level in the SCR catalytic converter is regulated in a closed-loop to a specified reagent substance set point fill stand. 
     Provision is made in one embodiment for the temperature of the SCR catalytic converter and the reagent substance fill level in the SCR catalytic converter and/or the space velocity of the exhaust gas in the SCR catalytic converter to be taken into account along with the NO x  emissions before the SCR catalytic converter when calculating the NO x  concentration arising downstream after the SCR catalytic converter. In so doing, a high degree of accuracy is achieved. 
     Provision is made in an additional embodiment for the specified reagent substance set point fill level to be established at least at a maximum value, which corresponds to an SCR catalytic converter completely filled with the reagent substance. This embodiment has advantages in connection with an additional embodiment, in which provision is made for the NO x  sensor to have a cross sensitivity with regard to the reagent substance. The cross sensitivity can thereby specifically be taken advantage of; in that when a difference occurs, initially a manipulation of the reagent substance signal is continuously conducted for the purpose of reducing the dosage of the reagent substance. This takes place because the difference reflects a reagent substance slip with a high degree of probability. 
     The device according to the invention to operate an internal combustion engine concerns initially a control unit, which is designed to implement the procedure. The control unit especially contains a difference ascertainment, which ascertains the difference between the exhaust gas sensor signal provided by the NO x  sensor and the NO x  concentration downstream after the SCR catalytic converter, which is calculated by a NO x  concentration ascertainment. Provision is additionally made for the control unit to contain a characteristic diagram, which provides a correction signal for the manipulation of the reagent substance signal and in which the correction signals are deposited at least as a function of the difference and as a function of the measurement for the temperature of the SCR catalytic converter. 
     The control unit preferably contains an electrical storage memory, in which the procedural steps are filed as a computer program. 
     Additional advantageous modifications and embodiments of the procedural approach according to the invention result from additional dependent claims and from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The FIGURE shows a technical environment, in which the procedure according to the invention is operating. 
     
    
    
     DETAILED DESCRIPTION 
     The FIGURE shows an internal combustion engine  10 , in whose air intake section  11 , an air ascertainment  12  is disposed; and in whose exhaust gas section  13 , a reagent substance metering  14 , a first NO x  sensor  15 , an SCR catalytic converter  16 , a temperature sensor  17  assigned to the SCR catalytic converter  16  as well as a second NO x  sensor  18  are disposed. 
     An exhaust gas flow ms_abg as well as an untreated NO x  concentration NO x     —   vK arises downstream after the internal combustion engine  10 . A NO x  concentration NO x     —   nK as well as a reagent substance slip ms_Rea_nK arises downstream after the SCR catalytic converter  16 . 
     The air ascertainment  12  provides an air signal ms_L to a control unit  20 ; the internal combustion engine  10  provides an engine rotational speed signal n; the first NO x  sensor  15  provides a first NO x  signal NO x     —   vK_mess; the temperature sensor  17  provides a measurement for the temperature te_Kat of the SCR catalytic converter  16 ; and the second NO x  sensor  18  makes an exhaust gas sensor signal S_nK available to the control unit  20 . 
     The control unit  20  provides a fuel signal m_K to a fuel metering device  25  assigned to the internal combustion engine  10  as well as a reagent substance signal S_Rea to the reagent substance metering  14  and to the fuel metering device  25 . 
     The control unit  20  contains a torque ascertainment  30 , which is provided with the air signal ms_L, the engine rotational speed signal n as well as a torque set point MFa and which ascertains a torque Md of the internal combustion engine  10 . 
     The control unit  20  additionally contains an untreated NO x  concentration ascertainment  31 , which is provided with the air signal ms_L, the engine rotational speed signal n as well as the fuel signal m_K and which ascertains a calculated measurement NO x     —   vK_mod of the untreated NO x  concentration before the SCR catalytic converter NO x     —   vK. 
     The control unit  20  further contains a NO x  concentration ascertainment  32 , which is provided with the calculated measurement NO x     —   vK_mod for the untreated NO x  concentration before the SCR catalytic converter NO x     —   vK, the measurement for the temperature te_Kat of the SCR catalytic converter  16 , a space velocity RG as well as a reagent substance fill level ReaSp in the SCR catalytic converter  16 , and which ascertains a calculated measurement NO x     —   nK_mod for the NO x  concentration NO x     —   nK downstream after the SCR catalytic converter  16 . 
     The calculated measurement NO x     —   nK_mod for the NO x  concentration NO x     —   NK and the exhaust gas sensor signal S_nK are provided to a first difference ascertainment  33 , which ascertains a difference D. The difference D as well as the measurement for the temperature te_Kat is provided to a characteristic diagram  34 , which provides a correction signal d_ReaSp, which is supplied to a first summing amplifier  35 . 
     The first summing amplifier  35  ascertains a reagent substance actual fill level ReaSp_Ist from the correction signal d_ReaSp and the reagent substance fill level ReaSp. The reagent substance actual fill level ReaSp_Ist is provided to a second difference ascertainment  36 , which ascertains a control deviation  37  from the reagent substance actual fill level ReaSp_Ist and a reagent substance set point fill level ReaSp_Soll. A closed-loop controller  38  ascertains a regulating variable  39  from the control deviation  37 . This regulating variable  39  is provided to a second summing amplifier  40 , which adds a reagent substance pilot variable S_Rea_VS to the regulating variable  39  and which provides a reagent substance signal S_Rea. 
     The reagent substance pilot variable S_Rea_VS is provided by a pilot value ascertainment  41 , which ascertains the reagent substance pilot variable S_Rea_VS from the torque Md and the engine rotational speed signal n. 
     The reagent substance signal S_Rea is provided to a catalytic converter model  42 , which continues to obtain the untreated NO x , concentration before the SCR catalytic converter NO x     —   vK, the NO x  concentration after the SCR catalytic converter NO x     —   nK, the measurement for the temperature te_Kat of the SCR catalytic converter  16  and the reagent substance slip ms_Rea_nK. The catalytic converter model  42  supplies the reagent substance fill level ReaSp. 
     The procedure according to the invention works as follows: 
     The torque ascertainment  30  disposed in the control unit  20  ascertains the torque Md generated by the internal combustion engine  10  as a function of at least the specified torque set point MFa, which, for example, is provided by an unspecified accelerator pedal of a motor vehicle, in which the internal combustion engine  10  is disposed as the power source. The torque Md is at least approximately a measurement for the load of the internal combustion engine  10 . When ascertaining the torque Md, the engine rotational speed signal n and/or the air signal ms_L supplied by the air acquisition  12  can continue to be taken into account. 
     The control unit  20  transmits the fuel signal m_K, which is established especially on the basis of the torque, to the fuel metering device  25 . The fuel signal m_K establishes, for example, a fuel point of injection as well as an injected quantity of fuel. 
     The fuel combusted in the internal combustion engine  10  leads to the exhaust gas flow ms_abg, which can contain the undesirable, more or less large, untreated NO x  concentrations before the SCR catalytic converter NO x     —   vK as a function of the operating point of the internal combustion engine  10 . 
     At least the SCR catalytic converter  16  is disposed in the exhaust gas section  13  of the internal combustion engine  10  for the most extensive as possible removal of the untreated NO x  concentration before the SCR catalytic converter NO x     —   vK. Beside the SCR catalytic converter  16 , provision can be made for additional catalytic converters and/or a particle filter. The SCR catalytic converter  16  supports the reducing reaction of the NO x  with a reagent substance, which is either metered into the exhaust gas section  13  with the reagent substance metering  14  and/or if need be is supplied within the motor. Provision can be made for a source material instead of the reagent substance. In the case of the reagent substance ammonia, provision can be made, for example, for a urea-water solution or, for example, for ammonium carbamate to be the source material. The dosage is established with the reagent substance signal S_Rea, which is provided to the reagent substance metering  14 . Alternatively or additionally, when the reagent substance is supplied by the engine internally, the fuel signal m_K can be modified by the reagent substance signal S_Rea in such a way that the required amount of reagent substance is generated internally. 
     After starting the procedure according to the invention, the establishment of the reagent substance set point fill level ReaSp_Soll occurs at the specified reagent substance fill level ReaSp_Soll, which, for example, can be established at a value, which lies under the maximum possible reagent substance fill level in the SCR catalytic converter  16  if a reagent substance slip ms_Rea_nK is to be avoided if possible in all operating states. Provision is made in contrast in an advantageous embodiment for the specified reagent substance set point fill level ReaSp_Soll to correspond to the maximum possible reagent substance fill level in the SCR catalytic converter  16 , which is dependent on the temperature te_Kat in the SCR catalytic converter  16 . The correlation is described in detail in the German patent DE 10 2004 031 624 A1 mentioned at the beginning of the application, which is hereby referred to in its entirety. 
     The establishment of the reagent substance set point fill level ReaSp_Soll can also be accomplished at a value higher than the maximum value, so that in this case a reagent substance slip ms_Rea_nK, which at least is very small, has to always be anticipated. The important advantage of establishing the reagent substance set point fill level ReaSp_Soll at least the maximum value or at a higher value, which can only exist as an operand, lies with the fact that the SCR catalytic converter  16  is constantly operated in the range of its maximum efficiency, at which the highest possible NO x  conversion takes place. In all of the operating states of the internal combustion engine  10  as well as the SCR catalytic converter  16  and within all of the parameters of the exhaust gas in the exhaust gas section  13 , assurance is made for the NO x  concentration after the SCR catalytic converter NO x     —   nK to have the minimum possible value. A reagent substance slip ms_Rea_nK occurring at least occasionally must be taken for granted. Provided the reagent substance set point fill level ReaSp_Soll is established at a value higher than the maximum value, a small reagent substance slip ms_Rea_nK always occurs. 
     The reagent substance fill level ReaSp in the SCR catalytic converter  16  can be set by an open loop control to a specified reagent substance set point fill level ReaSp_Soll. Provision is preferably made for a closed-loop controlled setting to the specified reagent substance set point fill level ReaSp_Soll. In the second difference ascertainment  36 , the reagent substance set point fill level ReaSp_Soll is compared with the reagent substance actual fill level ReaSp_Ist. The second difference ascertainment  36  develops a difference, which is fed to the closed-loop controller  38  as a control deviation  37 . The closed-loop controller  38  then ascertains the regulating variable  39 . The regulating variable  39  is added to the preferably already existing reagent substance pilot variable S_Rea_VS in the second summing amplifier  40 . 
     The reagent substance pilot variable S_Rea_VS can specify, for example, a base amount of the reagent substance to be metered as a function of the operating parameters of the internal combustion engine  10 . In the pilot variable ascertainment  41 , the torque Md as well as the engine rotational speed signal n is, for example, taken into account. This procedural approach allows for a comparatively easy application. 
     The regulating variable  39 , which if necessary is combined with the existing reagent substance pilot variable S_Rea_VS, establishes the reagent substance signal S_Rea, which is fed to the reagent substance metering  15  and/or the fuel metering device  25 . The reagent substance signal S_Rea enables, for example, a cross section of a valve to open, which corresponds to a specified reagent substance flow, which additionally depends on the reagent substance pressure. 
     The catalytic converter model  42  ascertains the reagent substance fill level ReaSp using the reagent substance signal S_Rea while taking into account the untreated NO x  concentration before the SCR catalytic converter NO x     —   vK, the NO x  concentration after the SCR catalytic converter NO x     —   nK as well as the measurement for the temperature te_Kat of the SCR catalytic converter  16 . The reagent substance slip ms_Rea_nK is if need be additionally taken into account. The catalytic converter model  42  is described in the aforementioned State of the Art, to which reference is again made at this point. 
     Provision is made to ascertain the calculated measurement NO x     —   nK_mod for the NO x  concentration after the SCR catalytic converter NO x     —   nK. The calculation takes place in the NO x  concentration ascertainment  32  on the basis of the calculated measurement NO x     —   vK_mod for the untreated NO x  concentration before the SCR catalytic converter NO x     —   vK, which the untreated NO x  concentration ascertainment  31  supplies on the basis of, for example, the torque Md and/or the engine rotational speed signal n. The NO x  concentration ascertainment  32  ascertains a degree of efficiency for the SCR catalytic converter  16  using the measurement for the temperature te_Kat, which the temperature sensor  17  provides. The temperature sensor  17  can be disposed before, in or downstream after the SCR catalytic converter  16 , so that the sensor signal supplied by the temperature sensor  17  is at least approximately a measurement for the temperature te_Kat of the SCR catalytic converter  16 . Provision can also be made for an estimate of the temperature te_Kat of the SCR catalytic converter  16  instead of a temperature measurement. 
     Preferably the exhaust gas space velocity RG is additionally taken into account in the NO x  concentration ascertainment  32 . The exhaust gas space velocity RG can be ascertained from the known geometric data of the SCR catalytic converter  16  and from the exhaust gas flow ms_abg. Additionally, the reagent substance fill level ReaSp in the SCR catalytic converter  16  is taken into account because the degree of efficiency also especially depends on the reagent substance fill level ReaSp. 
     The measurement NO x     —   nK_mod calculated by the NO x  concentration ascertainment  32  for the NO x  concentration after the SCR catalytic converter NO x     —   nK is subtracted from the exhaust gas sensor signal S_nK in order to obtain the difference D. A difference D, which occurs, can be taken into account in the metering strategy, and the reagent substance signal S_Rea can be manipulated accordingly. 
     When the correction signal d_ReaSp is being supplied, the measurement for the temperature te_Kat of the SCR catalytic converter is furthermore taken into account along with the difference D. The required correction signals d_ReaSp are deposited in the characteristic diagram  34  at least as a function of the difference D and as a function of the measurement for the temperature te_Kat of the SCR catalytic converter  16 . The characteristic diagram  34  is addressed as a function at least of both parameters and emits the correction signal d_ReaSp corresponding to the deposited value. Within the scope of the patent application, the characteristic diagram  34  is provided with at least pairs of variates; whereby when the difference D is constant at higher temperatures te_Kat, the dosage of the reagent substance is to be reduced by a slight degree lower than the dosage at lower temperatures te_Kat. Correspondingly when the difference D is constant at higher temperatures te_Kat, the reagent substance fill level ReaSp is to be increased by a slight degree than is the case at lower temperatures. 
     The correction signal d_ReaSp could immediately be pulled up to influence the reagent substance signal S_Rea. In the example of embodiment depicted, the reagent substance signal S_Rea is indirectly affected by an intervention into the reagent substance fill level ReaSp, whereby the correction signal d_ReaSp manipulates the reagent substance actual fill level ReaSp_Ist in the SCR catalytic converter  16 . The reagent substance fill level ReaSp calculated from the catalytic converter model  42  is acted upon by the correction signal d_ReaSp, so that the reagent substance actual fill level ReaSp is modified. Provided that a difference D corresponding to a reagent substance slip ms_Rea_nK occurs, an increase in the reagent substance actual fill level ReaSp_Ist, for example, results, which due to the closed-loop control consequently elicits a degradation of the reagent substance signal S_Rea. 
     It is possible in principle to acquire the NO x  concentration NO x     —   nK after the SCR catalytic converter with a NO x  sensor and to acquire the reagent substance slip ms_Rea_nk with a reagent substance sensor. The utilization of an existing cross sensibility of the second NO x  sensor  18  with regard to the reagent substance is, however, particularly advantageous or a targeted development of such a cross sensibility. In this case, the exhaust gas sensor signal S_nK reflects the sum of the reagent substance slip ms_Rea_nK and the NO x  concentration after the SCR catalytic converter NO x     —   nk. A difference D, which occurs, could mean for that reason that either a reagent substance slip ms_Rea_nK or a high NO x  concentration after the SCR catalytic converter NO x  nK has occurred. Discrimination between the two would not be possible in this operating state. If according to the advantageous embodiment the reagent substance set point fill level ReaSp_Soll in the SCR catalytic converter  16  is established to the maximum possible value, it can thereby be assumed that an overdosage of the reagent substance exists, which corresponds to a reagent substance slip ms_Rea_nK. Provision is made for the closed-loop control in the example of embodiment depicted, so that the reagent substance slip ms_Rea_nK either only occurs for a short time or in the case of a continuous overdosing of the reagent substance is limited to a small amount.