Abstract:
A method for operating a catalytic converter used for purifying the exhaust gas of an internal combustion engine, and a device for implementing the method, which provide for an open-loop or closed-loop control of the reagent fill level in the catalytic converter to a predefined storage setpoint value. The targeted stipulation of the storage setpoint value ensures, on one hand, that in non-stationary states of the internal combustion engine, there is a sufficient quantity of reagent available for the completest possible removal of at least one unwanted exhaust-gas component, and on the other hand, a reagent slip is avoided.

Description:
BACKGROUND INFORMATION 
   German Patent Application No. DE 101 39 142 describes an exhaust-gas treatment system of an internal combustion engine, in which, to reduce the NOx emissions, an SCR (selective catalytic reduction) catalytic converter is used which reduces the nitrogen oxides contained in the exhaust gas to nitrogen using the reagent ammonia. The ammonia is obtained in a hydrolysis catalytic converter, situated upstream from the SCR catalytic converter, from a urea-water solution. The hydrolysis catalytic converter converts the urea contained in the urea-water solution to ammonia and carbon dioxide. 
   In German Patent Application No. DE 197 39 848, a procedure is described by which the untreated NOx emissions of the internal combustion engine can be at least approximately calculated from known operating parameters of the internal combustion engine. The starting point is a family of characteristics which is defined (spanned) by the load and the speed of the internal combustion engine. In addition, corrections can be provided, for example, as a function of the air ratio lambda. 
   European Patent Application No. EP 1 024 254 describes an exhaust-gas treatment system of an internal combustion engine, in which an SCR catalytic converter is likewise used for reducing NOx emissions. Ammonia is again provided as a reagent, which is obtained in the exhaust duct from a urea-water solution. The reagent rate is set on the basis of the fuel injection quantity and the speed of the internal combustion engine, as well as on the basis of at least one characteristic of the exhaust gas, e.g. the exhaust-gas temperature. 
   In European Patent Application No. EP 697 062, a method and a device are described for the controlled introduction of a reagent into an exhaust gas containing nitrogen oxide. An SCR catalytic converter is likewise provided which, as a reagent, needs ammonia that is obtained from a reagent introduced into the exhaust duct upstream of the SCR catalytic converter. At least one operationally-relevant parameter of the exhaust gas, at least one operationally-relevant parameter of a catalytic converter and optionally one operationally-relevant parameter of an internal combustion engine are acquired for determining the untreated NOx emissions of the internal combustion engine. In accordance with the ascertained untreated NOx emissions, an intermediate value is determined for a reagent rate to be stipulated, which is reduced by a reagent rate desorbed by the catalytic converter or is increased by a reagent rate adsorbed by the catalytic converter. 
   An object of the present invention is to provide a method for operating a catalytic converter used for purifying the exhaust gas of an internal combustion engine and a device for implementing the method which avoid overdosage and underdosage of the reagent. 
   SUMMARY OF THE INVENTION 
   The procedure of the present invention provides for open-loop or closed-loop control (regulation) of the reagent stored in a catalytic converter to a predefined storage setpoint value. The targeted stipulation of the storage setpoint value has the advantage that in non-stationary states of the internal combustion engine, on one hand there is a sufficient quantity of reagent available for the completest possible removal of at least one unwanted exhaust-gas component, and on the other hand, a reagent slip is avoided. Synonymous with the closed-loop or at least open-loop control to the predefined storage setpoint value is the closed-loop or at least open-loop control of the degree of saturation of the catalytic converter with the reagent. The degree of saturation corresponds to the ratio of the instantaneous adsorbed reagent quantity to the maximum possible reagent fill level of the catalytic converter. 
   One refinement provides that the storage setpoint value is a function of a measure for the temperature of the catalytic converter. This refinement takes into account the temperature dependence of the catalytic-converter storage capacity. One further development provides that, below an operating-temperature range of the catalytic converter toward lower temperatures, the temperature-dependent storage setpoint value is reduced. This further development takes into account the fact that the catalytic activity in the catalytic converter decreases as lower temperatures are approached. Another further development provides that, after a maximum lying within the operating-temperature range of the catalytic converter toward higher temperatures, the temperature-dependent storage setpoint value is reduced. This further development ensures that the maximum for the reagent fill level lies within the operating-temperature range of the catalytic converter, and that the decreasing reagent storage capacity of the catalytic converter, as higher temperatures are approached, is taken into account. 
   One development provides that a storage actual value reflecting the reagent fill level of the catalytic converter is ascertained at least on the basis of the NOx mass flow passing into the catalytic converter. In another embodiment, a storage actual value reflecting the reagent fill level is ascertained at least on the basis of an NOx mass flow leaving the catalytic converter. Consideration of the NOx mass flow passing into and/or leaving the catalytic converter permits comparatively simple ascertainment of the reagent fill level of the catalytic converter, since the NOx mass flows may be calculated on the basis of known operating parameters of the internal combustion engine and/or of the exhaust gas and/or of the catalytic converter. 
   One further refinement provides for calculation of a storage actual value reflecting the reagent fill level. The calculation is carried out on the basis of the reagent mass flow passing into the catalytic converter, reduced by the difference between the NOx mass flow passing into the catalytic converter and the NOx mass flow leaving the catalytic converter, further reduced by the reagent slip. 
   The device of the present invention relates to a data carrier on which the method of the present invention is stored as software. The device of the present invention also relates to a control unit of an internal combustion engine in which the method of the present invention is stored. The software may be brought directly or via a long-distance data transmission (Internet) onto the data carrier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a technical environment in which a method of the present invention proceeds. 
       FIG. 2  shows a structure of a control loop. 
       FIG. 3  shows a reagent fill level as a function of the temperature. 
       FIG. 4  shows a model of a catalytic converter. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an internal combustion engine  10 , in whose intake region an air sensor  11  is arranged, and in whose exhaust region a first NOx sensor  12 , a reagent-introducing device  13 , a catalytic converter  14  and a second NOx sensor  15  are arranged. A fuel-metering device  20  is assigned to internal combustion engine  10 , and a temperature sensor  21  is assigned to catalytic converter  14 . 
   The air sensor provides an air signal dmL to a control unit  30 . Internal combustion engine  10  emits a speed N to control unit  30 . First NOx sensor  12  provides a first NOx signal NOxvK and second NOx sensor  15  provides a second NOx signal NOxhK to control unit  30 . Temperature sensor  21  supplies a temperature signal Tp. Moreover, a torque setpoint value MFa, derived from an accelerator pedal (not shown) of a motor vehicle (likewise not further shown) is sent to control unit  30 . 
   Control unit  30  emits a fuel signal mE to fuel-metering device  20 . Control unit  30  triggers a reagent dosing valve  31  using a dosing signal qRea. 
     FIG. 2  shows a setpoint input unit  40  which emits a storage setpoint value NH3SpSW to a first summing unit  41  that forms the difference between storage setpoint value NH3SpSW and a storage actual value NH3Sp. System deviation  42  made available by first summing unit  41  is processed in a controller  43  to form a manipulated variable  44  that is fed to a second summing unit  45 . Second summing unit  45  adds manipulated variable  44  to a pre-control variable  46  and supplies dosing signal qRea which acts upon reagent-dosing valve  31 . 
   Dosing valve  31  releases a reagent flow NH3dmE, passing into catalytic converter  14 , which is an input variable of a catalytic-converter model  47  that provides storage actual value NH3Sp. 
   Temperature signal Tp is made available to setpoint-input unit  40 , controller  43  and catalytic-converter model  47 . 
     FIG. 3  shows a maximum possible reagent fill level  50 , as well as storage setpoint value NH3SpSW as a function of the temperature. 
     FIG. 4  shows catalytic-converter model  47  which is supplied with reagent flow NH3dmE streaming in, an NOx mass flow NOxdmE, which is related to the reagent, that flows into catalytic converter  14 , an NOx mass flow NOxdmA, which is related to the reagent, that leaves catalytic converter  14 , and a reagent slip NH3msAus. Catalytic-converter model  47  provides storage actual value NH3Sp. In addition, temperature signal Tp and/or the estimated efficiency of catalytic converter  14  may be fed to catalytic-converter model  47 . 
   The method according to the present invention functions as follows: 
   As a function at least of torque setpoint value MFa and/or as a function of speed N and/or as a function of air signal dmL, control unit  30  shown in  FIG. 1  stipulates fuel signal mE which determines the fuel quantity metered to internal combustion engine  10  by fuel-metering device  20 . The at least one catalytic converter  14  disposed in the exhaust region of internal combustion engine  10  is provided to eliminate at least one exhaust-gas component of internal combustion engine  10 . In the exemplary embodiment shown, the catalytic converter is in the form of an SCR catalytic converter which is intended to eliminate as completely as possible the untreated NOx emissions emitted by internal combustion engine  10 . According to type models presently available, SCR catalytic converter  14  needs a reagent which can be introduced as such or in the form of a precursor into the exhaust-gas flow upstream of catalytic converter  14 . To that end, reagent-introducing device  13  is provided which may optionally be identical with dosing valve  31 . As precursor for the reagent, a urea-water solution is provided, for example, which is converted into ammonia upstream of catalytic converter  14  or in catalytic converter  14  by thermolysis and hydrolysis. Alternatively, ammonia may be provided directly as reagent. The ammonia may also be obtained from ammonium carbamate. 
   The ammonia reagent reacts in SCR catalytic converter  14  with nitrogen oxides to form nitrogen and water. Dosing signal qRea may be stipulated, for example, at least as a function of the load status of internal combustion engine  10  and/or as a function of engine speed N. A measure for the load status of internal combustion engine  10  is, for instance, torque setpoint value MFa or fuel signal mE. If the dosing of the reagent is too low, the untreated NOx emissions of the internal combustion engine would be only partially eliminated. An overdosing is to be avoided, since a reagent breakthrough occurs downstream of catalytic converter  14 . 
   Catalytic converter  14  has a storage capacity with respect to the reagent. According to the present invention, it is provided to control in closed loop, or at least to control in open loop, the reagent fill level of catalytic converter  14  to the predefined storage setpoint value NH3SpSW. In the exemplary embodiment, a closed-loop control is assumed whose structure is shown in  FIG. 2 . Synonymous with the closed-loop or at least open-loop control to predefined storage setpoint value NH3SpSW is the closed-loop or at least open-loop control of the degree of saturation of catalytic converter  14  with the reagent. The degree of saturation corresponds to the ratio of the instantaneous adsorbed reagent quantity—the storage actual value NH3Sp—to the maximum possible reagent fill level  50  of catalytic converter  14 . 
   Storage setpoint value NH3SpSW, stipulated by setpoint input unit  40 , is compared in first summing unit  41  to storage actual value NH3Sp made available by catalytic-converter model  47 . First summing unit  41  forms the difference which is fed as system deviation  42  to controller  43 , which from it, ascertains manipulated variable  44 . System deviation  42  is also supplied to controller  43  for influencing the controller characteristics. If controller  43  is a PI controller, system deviation  42  is able to influence the P (proportional) component and/or the I (integral-action) component. For example, a complete cutoff of the P component may be provided if system deviation  42  exceeds a predefined threshold value. Provision may also be made that, in the event of a negative system deviation, manipulated variable  44  always has a predefined amount that corresponds to a minimum dosing signal qRea. This measure takes into account that reagent-dosing valve  31  cannot dose arbitrarily small reagent amounts. 
   In second summing unit  45 , manipulated variable  44  is added to optionally available pre-control variable  46 . Optionally formed pre-control variable  46  may predefine, for example, a basic quantity of the reagent to be dosed as a function of operating parameters of internal combustion engine  10 . Manipulated variable  44 , which is optionally linked with available pre-control variable  46 , stipulates dosing signal qRea which is sent to reagent-dosing valve  31 . Dosing signal qRea releases an opening cross-section of reagent-dosing valve  31  that corresponds to a predefined reagent flow rate, which is furthermore a function of the reagent pressure. 
   The reagent arrives, via reagent-introducing device  13 , at the exhaust region of internal combustion engine  10  upstream of catalytic converter  14 . Compressed air may be admixed if desired. Depending on the implementation, reagent-dosing valve  31  and reagent—introducing device  13  may coincide. Reagent flow NH3dmE passing into catalytic converter  14  is taken into account as an input variable of catalytic-converter model  47 . 
   Setpoint input unit  40  stipulates storage setpoint value NH3SpSW preferably as a function of at least one measure for the temperature of catalytic converter  14 . This refinement takes into account, on one hand, the temperature-dependent storage capacity of catalytic converter  14  with respect to the reagent, and on the other hand, the temperature-dependent catalytic activity. 
     FIG. 3  shows the maximum possible reagent fill level  50  in catalytic converter  14 . Maximum possible reagent fill level  50  decreases as the temperature rises. Setpoint input unit  40  stipulates storage setpoint value NH3SpSW in such a way that, in the event of a sudden sharp temperature increase, the desorbed NH3 quantity is able to bring a reaction to completion in catalytic converter  14  with the NOx quantities available, without generating a reagent slip NH3dmA. For example, the predefined difference between maximum possible reagent fill level  50  and storage setpoint value NH3SpSW should not drop below 20%. 
   A specification of the degree of saturation, which corresponds to the relationship of currently adsorbed reagent quantity to maximum possible reagent fill level  50 , corresponds to the specification of storage setpoint value NH3SpSW. 
   The consideration of the measure for the temperature of catalytic converter  14  also plays an important role. Temperature sensor  21  provides the measure for the temperature temp (Tp) of catalytic converter  14 . In the exemplary embodiment shown, temperature sensor  21  is allocated directly to catalytic converter  14 . In one practical implementation, temperature sensor  21  may be disposed upstream of catalytic converter  14 , in particular downstream of catalytic converter  14 , as well as at a suitable location within catalytic converter  14 . In another embodiment, at least two temperature sensors may be provided at different locations. Another possibility provides for the calculation of at least one measure for temperature temp (Tp) of catalytic converter  14  on the basis of operating parameters of internal combustion engine  10  and/or characteristics of the exhaust gas and/or of catalytic converter  14  itself. 
   Setpoint input unit  40  takes into account the decrease in catalytic activity in catalytic converter  14  by a reduction of storage setpoint value NH3SpSW toward lower temperatures. A maximum of storage setpoint value NH3SpSW is obtained which essentially lies at the lower border of the operating-temperature range of catalytic converter  14 . 
   The measure for temperature Tp of catalytic converter  14  is furthermore fed to controller  43  for influencing the P component and/or I component. This refinement takes into account that controller  43  may be at least partially or completely switched off if there is a drop below a predefined lower temperature limit. 
   Storage actual value NH3Sp is ascertained by catalytic-converter model  47  at least in light of reagent flow NH3dmE passing into catalytic converter  14 . Moreover, NOx mass flow NOxdmE streaming into catalytic converter  14 , corresponding to the untreated NOx emissions of internal combustion engine  10 , is preferably taken into account. To simplify the calculations, NOx mass flow NOxdmE passing into catalytic converter  14  can be related to the reagent NH3. Furthermore, NOx mass flow NOxdmA leaving catalytic converter  14  is preferably taken into account and is likewise expediently related to the reagent NH3. Catalytic-converter model  47  forms the difference between NOx mass flow NOxdmE flowing into and NOx mass flow NOxdmA leaving catalytic converter  14 . 
   Catalytic converter model  47  optionally may also take into account the reagent slip NH3dmA, which, however, may be omitted to simplify the calculation of the reagent fill level corresponding to storage actual value NH3Sp. Moreover, if desired, temperature signal Tp and/or the calculated efficiency of catalytic converter  14  may be considered. 
   A change in storage actual value NH3Sp, corresponding to a change in the reagent fill level, may be calculated as follows:
 
 dNH 3 Sp=NH 3 dmE −( NOxdmE ( NH 3-specific)− NOxdmA ( NH 3-specific))− NH 3 dmA.  
 
   The reagent fill level corresponding to storage actual value NH3Sp is yielded by ascertaining the time integral. 
   The preferably NH3-specific NOx mass flow NOxdmA leaving catalytic converter  14  may alternatively be ascertained in light of the catalytic-converter efficiency. In this case, it is possible to take into account the measure for temperature Tp of catalytic converter  14  and/or storage actual value NH3Sp and/or the exhaust-gas velocity and/or the feed ratio alpha, which is given by the reagent flow NH3dmE flowing in relative to the NOx mass flow NOxdmE flowing in. 
   The preferably NH3-specific NOx mass flow NOxdmE passing into catalytic converter  14  and/or the preferably likewise NH3-specific NOx mass flow NOxdmA leaving catalytic converter  14  may be calculated on the basis of operating parameters of internal combustion engine  10  and/or characteristics of the exhaust gas. In the exemplary embodiment shown, to detect NOx mass flow NOxdmE passing into catalytic converter  14 , first NOx sensor  12  is provided which makes available first NOx signal NOxvK. First NOx sensor  12  detects the NOx concentration in the exhaust gas, which must be set off against the exhaust-gas mass flow to obtain the NOx mass flow. In the exemplary embodiment shown, to detect the preferably NH3-specific NOx mass flow NOxdmA leaving catalytic converter  14 , second NOx sensor  15  is provided which makes available the second NOx signal NOxhK. Second NOx sensor  15  detects the NOx concentration in the exhaust gas, which again must be set off against the exhaust-gas mass flow to obtain the NOx mass flow. 
   An alternative form of the ascertainment of storage actual value NH3Sp provides for the use of a Lunberg observer which ascertains storage actual value NH3Sp from state variables of catalytic-converter model  47 . In this case, catalytic converter  14  to be observed is modeled and the model receives the same input variables as the real system. Deviations between the real and the modeled output variables are fed back as correction via a feedback structure into the modeled system. The input variables for catalytic-converter model  47  may, for example, be reagent flow NH3dmE passing into catalytic converter  14 , NOx mass flow NOxdmE passing into catalytic converter  14  as well as the air ratio Lambda in the exhaust gas. Temperature Tp of catalytic converter  14 , NOx mass flow NOxdmA leaving catalytic converter  14  as well as reagent slip NH3dmA are provided, for example, as output variables.