Abstract:
A method for providing a specification about an efficiency of a cooler for recirculated exhaust gas in an internal combustion engine includes: measuring a temperature of the recirculated exhaust gas cooled by the cooler; and ascertaining the specification about the efficiency of the cooler as a function of the measured temperature of the cooled recirculated exhaust gas. With the aid of the specification about the efficiency of the cooler it is possible to determine a failure of the cooler or to calculate a temperature of the recirculated exhaust gas in order implement an engine control system of the internal combustion engine on the basis of a reliable temperature specification that is not influenced by lag effects.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to Application No. 10 2008 001 418.4, filed in the Federal Republic of Germany on Apr. 28, 2008, which is expressly incorporated herein in its entirety by reference thereto. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to engine systems for internal combustion engines having an exhaust gas recirculation system, which have a cooler in the exhaust gas recirculation segment for cooling the recirculated exhaust gas. 
       BACKGROUND INFORMATION 
       [0003]    In engine systems for internal combustion engines, exhaust gas is recirculated in order to reduce the nitrogen oxide component of the exhaust gas. Due to the increased soot component in the exhaust gas brought about by the exhaust gas recirculation, the quantity of recirculated exhaust gas (indicated by the exhaust gas recirculation rate: EGR rate) is limited with the aid of a constant exhaust gas recirculation rate as a compromise between the nitrogen oxide component and the soot component in the exhaust gas. 
         [0004]    The recirculated exhaust gas is normally conducted through a cooler (EGR cooler) for cooling. Cooling the recirculated exhaust gas allows for a higher EGR rate at a constant intake manifold pressure and may thus have a significant influence on minimizing raw emissions. 
         [0005]    Modern control arrangements such as, e.g., the model-based charge control (MCC) make it possible to control the EGR rate and have the advantage over conventional air mass controllers of being able to keep the emissions of the internal combustion engine in narrower tolerances. The required control variable of the EGR rate is generally calculated with the aid of an air system model which assumes the model of an intact, ideal cooler for modeling the temperature of the cooled exhaust gas. 
         [0006]    The efficiency of the EGR cooler, however, may change while the internal combustion engine is in operation such that the cooling performance varies. Due to the changed density of the recirculated exhaust gas, the variation of the cooling performance results in a change of the EGR rate and may thus result in a significant fluctuation of the emission of the internal combustion engine. 
         [0007]    Furthermore, because the cooler for cooling the recirculated exhaust gas (EGR cooler) is relevant in terms of emissions, the law requires that the cooling function be monitored in connection with the on-board diagnosis. 
       SUMMARY 
       [0008]    Example embodiments of the present invention provide a method for determining an efficiency specification of an EGR cooler. 
         [0009]    Example embodiments of the present invention detect a malfunction of the EGR cooler. 
         [0010]    Example embodiments of the present invention provide an EGR rate control such that the fluctuations of the nitrogen oxide emissions are reduced. 
         [0011]    According to example embodiments of the present invention, a method is provided for furnishing a specification about an efficiency of a cooler for recirculated exhaust gas in an internal combustion engine. The method includes the following: measuring a temperature of the recirculated exhaust gas cooled by the cooler; and ascertaining the specification about the efficiency of the cooler as a function of the measured temperature of the cooled recirculated exhaust gas. 
         [0012]    An absolute value of the efficiency may be ascertained as the specification about the efficiency of the cooler. Alternatively, the specification about the efficiency of the cooler may also be ascertained as a specification about a change in the efficiency of the cooler with respect to a reference efficiency of the reference cooler and furthermore as a function of the cooler model temperature, the cooler model temperature of the cooled recirculated exhaust gas being determined according to a cooler model for a reference cooler as a function of a mass flow of the recirculated exhaust gas. 
         [0013]    According to example embodiments of the present invention, a method for detecting a failure of the cooler is provided, a specification about the efficiency of a cooler being ascertained in accordance with the above method, the failure of the cooler being determined as a function of a threshold value. 
         [0014]    A method for operating an internal combustion engine may be provided, in which an engine control system setting an exhaust gas recirculation rate for the internal combustion engine as a function of a provided temperature of the cooled recirculated exhaust gas, the provided temperature of the cooled recirculated exhaust gas being determined as a function of the specification about the efficiency of the cooler, which is ascertained in accordance with the above method. 
         [0015]    Example embodiments of the present invention provide a method for determining a specification about an efficiency of an EGR cooler, e.g., a specification about an absolute efficiency or a change in the efficiency of the cooler such that with the aid of the specification about the efficiency it is possible to detect a failure of the EGR cooler. Furthermore, an engine control system may be operated as a function of the temperature of the recirculated exhaust gas, the temperature of the recirculated exhaust gas being provided, not by the temperature measured by a temperature detector in the recirculation line, but rather by a temperature calculated from the specification about the efficiency. 
         [0016]    Furthermore, the efficiency of an intact cooler may also change within certain limits, and it thus affects the temperature of the exhaust gas behind the EGR cooler and thus the EGR rate. The change in the efficiency is caused, for example, by soot deposits in the EGR cooler. Such soot deposits, however, may again dissipate during certain operating phases such that changes of the cooler efficiency result over the life of a vehicle. While, in current air system models, a constant, permanently specified cooler model is always assumed, example embodiments of the present invention provide for the monitoring function to correct or adapt the cooler model by the calculated change in efficiency Δη cooler  or based on the absolute efficiency η cooler . The adaptation preferably occurs only in small steps. This is sensible because changes in the efficiency of the cooler may also be observed only over longer time periods. This measure results in a qualitatively better modeled temperature of the recirculated exhaust gas behind the EGR cooler and thus also to a more precise calculation of the EGR rate. Additionally, the adaptation of the cooler efficiency ensures that the quality of the dynamics of the temperature specification, which is calculated via the efficiency, is good, in contrast to the dynamics of the temperature detector. 
         [0017]    Furthermore, the specification about the efficiency may be low-pass filtered. In particular, the time constant of the low-pass filtering may be performed as a function of an enabling time, the enabling time indicating the total time during which one or more enabling conditions are satisfied. 
         [0018]    According to example embodiments, the specification about the efficiency may be ascertained as a function of one or more enabling conditions. The enabling conditions may include: the exhaust gas temperature exceeds a certain predetermined exhaust gas threshold temperature; an exhaust gas mass flow of the recirculated exhaust gas exceeds a certain predetermined exhaust gas mass flow threshold value; and the EGR rate exceeds a certain predetermined EGR rate threshold value. 
         [0019]    Furthermore, if the specification about the efficiency of the cooler is ascertained as the specification about a change in the efficiency of the cooler with respect to a reference efficiency, then it is possible to determine the threshold value as a function of an exhaust gas mass flow through the cooler in accordance with a cooler model. 
         [0020]    The cooler model is able to describe a correlation between the efficiency of the cooler and the exhaust gas mass flow through the cooler, the cooler model being adapted as a function of an ascertained efficiency of the cooler in a particular exhaust gas mass flow. In particular it is possible to perform the adaptation of the cooler model by interpolation as a function of a cooler model of the reference cooler and a cooler model of a suppressed cooler. 
         [0021]    According to example embodiments, an engine control unit is provided for furnishing a specification about an efficiency of a cooler for recirculated exhaust gas in an internal combustion engine. The engine control unit is arranged: to receive a specification about a temperature of the recirculated exhaust gas cooled by the cooler; and to ascertain the specification about the efficiency of the cooler as a function of the temperature of the cooled recirculated exhaust gas. 
         [0022]    According to example embodiments, the engine control unit may be provided for detecting a failure of the cooler; the engine control unit being adapted to ascertain a specification about the efficiency of a cooler according to the above method and to determine a failure of the cooler as a function of a threshold value. 
         [0023]    According to another aspect, a computer program product is provided having program code for carrying out the above method when the program is executed in an engine control unit. 
         [0024]    Example embodiments of the present invention are explained in greater detail in the following text on the basis of the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic representation of an engine system having an exhaust gas recirculation system; 
           [0026]      FIG. 2  is a schematic block diagram for representing a method for monitoring the function of the EGR cooler and for providing an efficiency correction value; 
           [0027]      FIG. 3  is a schematic representation that illustrates the adaptation of the conventional cooler model by an ascertained change in the efficiency of the EGR cooler; 
           [0028]      FIG. 4  is a schematic representation of a method for monitoring the function of an EGR cooler on the basis of an ascertained absolute efficiency of the EGR cooler; and 
           [0029]      FIG. 5  is a representation of the dependence of the cooler efficiency on an EGR mass flow of the recirculated exhaust gas including an illustration of an estimation or interpolation of the existing efficiency at a certain EGR mass flow. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  shows a schematic representation of an engine system  1  having an internal combustion engine  2  that has four cylinders  3 . Via corresponding intake valves (not shown), an air supply  4 , e.g. in the form of an intake manifold, supplies air, required for the combustion, to cylinders  3  of internal combustion engine  2 . The exhaust gas produced by the combustion in cylinders  3  is removed from internal combustion engine  2  via an exhaust branch  5 . 
         [0031]    Between exhaust branch  5  and supply  4 , an exhaust gas recirculation line  6  is provided, which has an exhaust gas recirculation valve  7  (EGR valve) in order to feed a portion of the exhaust gas removed through exhaust branch  5  into supply  4 . EGR valve  7  is variably adjustable in order to implement a desired exhaust gas recirculation rate (EGR rate) in engine system  1 . The EGR rate is defined as the ratio between the exhaust gas mass flow m EGR  through recirculation line  6  and the total mass flow m 22  of the gas quantity flowing into cylinders  3  of internal combustion engine  2 . The gas quantity flowing into cylinders  3  is determined by the sum of the air mass flow m 21  drawn in by internal combustion engine  2  and the recirculated exhaust gas flow m EGR . 
         [0032]    Exhaust gas is recirculated into air supply  4  so as to reduce the nitrogen oxide produced by the combustion in cylinders  3 . The EGR rate is controlled to be substantially constant in accordance with an exhaust gas recirculation control system that controls EGR valve  7  as a function of exhaust gas values, combustion and/or operating parameters of internal combustion engine  3 . In addition to the aspirated air mass flow m 21 , the exhaust gas recirculation control system also takes the temperature T EGR  of the recirculated exhaust gas (EGR temperature) into account since the latter affects the density of the exhaust gas. In this regard, it is particularly desirable to reduce the temperature of the exhaust gas such that the desired EGR rate, determined by the exhaust gas recirculation control system, may be increased without reducing the aspirated air mass flow m 21 . 
         [0033]    Therefore, an exhaust gas cooler  8  (EGR cooler) is provided between exhaust branch  5  and EGR valve  7 . EGR cooler  8  cools the exhaust gas flowing through recirculation line  6  with the aid of cooling water or the like. Temperature T EGR  of the cooled recirculated exhaust gas is measured with the aid of a temperature detector  9  situated between EGR cooler  8  and EGR valve  7 . Furthermore, the temperature of the exhaust gas flowing into EGR cooler  8  is indicated by T 3 , and the temperature of the cooling water used for cooling in EGR cooler  8  is indicated by T cooling water . The temperature T cooling water  of the cooling water may be ascertained e.g. using a suitable cooling water temperature detector (not shown). 
         [0034]    After leaving internal combustion engine  3 , exhaust gas temperature T 3  of the exhaust gas is measured either by another temperature detector (not shown) or is determined according to a model from operating parameters such as injection quantity, the temperature of the mass flow taken into cylinders  3  via the intake valves and other operating parameters such as e.g. rotational speed, load torque, ignition timing, etc., in accordance with a characteristics map or an underlying function of an engine model. 
         [0000]        T   3   =f ( T   22 ,injection quantity,etc.) 
         [0000]    where T 22  corresponds to the temperature of the gas (air, exhaust gas) fed into the internal combustion engine, which results from the temperature T 21  of the air aspirated from the surroundings, the EGR temperature T EGR  and the EGR rate. 
         [0035]      FIG. 2  shows a block diagram for representing a function for ascertaining an efficiency correction value Δη cooler     —     correction  and for establishing a failure of EGR cooler  8  schematically in a block diagram. 
         [0036]    In an efficiency change calculation unit  10 , a change in efficiency Δη cooler  is calculated as a function of the EGR temperature T EGR , which is measured by temperature detector  9 , as a function of a modeled temperature specification of the temperature value T EGR model  obtaining at the position of temperature detector  9  when EGR cooler  8  is intact, as a function of a temperature specification of the exhaust gas when leaving internal combustion engine  2  and as a function of the temperature of the cooling water T cooling water , in accordance with the following formula: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               
                 η 
                 cooler 
               
             
             = 
             
               
                 
                   T 
                   EGR 
                 
                 - 
                 
                   T 
                   EGRmodel 
                 
               
               
                 
                   T 
                   3 
                 
                 - 
                 
                   T 
                   
                     cooling 
                      
                     
                         
                     
                      
                     water 
                   
                 
               
             
           
         
       
     
         [0037]    The temperature value T EGR     —     model  obtaining in an intact EGR cooler  8  is provided by a cooler model  22 . 
         [0038]    Enabling conditions are checked in an enabling unit  11  and an enabling signal FS is generated if the enabling conditions obtain. The enabling conditions may include, for example, that the calculation of the change in efficiency of EGR cooler  8  is taken into account only if the temperature of exhaust gas T 3  exceeds a certain predetermined exhaust gas threshold temperature T 3     —     sw . Furthermore, one of the enabling conditions may be that the efficiency change calculation is performed only in the case of a sufficiently great exhaust gas mass flow dm EGR  through recirculation line  6  since otherwise the temperature T EGR  measured by temperature detector  9  does not allow for a sufficiently precise conclusion regarding the actual cooling power of EGR cooler  8 . Consequently, the exhaust gas mass flow dm EGR  flowing through recirculation line  6  is compared to a mass flow threshold value dm EGR     —     SW  or the EGR rate is compared to an EGR rate threshold value and enabling signal FS is activated only if the exhaust gas mass flow exceeds the mass flow threshold value or the EGR rate exceeds the EGR rate threshold value. Additional enabling conditions are conceivable as well. Generally, the enabling conditions are to ensure that the ascertained efficiency change Δη cooler  is taken into further consideration only if it is ensured with sufficient reliability that the deviation from the efficiency η cooler  of an intact cooler may be detected reliably and with a low error susceptibility. 
         [0039]    When enabling signal FS is activated, a counter is continuously incremented in a counter unit  12  as a function of a counting pulse clk, which thus specifies a total time t cumulative , during which enabling signal FS is activated. The counter value of counter unit  12  is compared in a comparator unit  13  to a counter threshold value ZSW and an output of comparator unit  13  is relayed to a debouncing unit  14 . The output of comparator unit  13  thus indicates by a logical level when the enabling conditions were/are satisfied for a minimum time period indicated by counter threshold value ZSW. 
         [0040]    Calculated efficiency change Δη cooler  is supplied to a low-pass filter  15 , the filter output value of low-pass filter  15  being calculated only if enabling signal FS indicates that the enabling conditions obtain, that is, only in favorable operating conditions of internal combustion engine  2 . A high filter time constant in the range of several seconds, minutes or even hours increases the robustness of the calculation such that even changes in the dynamics of temperature detector  9 , e.g. by soot deposits, have only a small effect on the resulting filtered efficiency change Δη cooler     —     filtered . 
         [0041]    If the total period of time, during which the enabling signal is activated, has exceeded the period of time indicated by counter threshold value ZSW, then the filtered efficiency change Δη cooler     —     filtered , which is supplied to debouncing unit  14 , is applied as a debounced efficiency change Δη cooler     —     debounced  to an additional comparator unit  16 . In the additional comparison unit  16 , the debounced efficiency change is compared to an efficiency change threshold value WGDS and an error is determined if the efficiency change is greater than efficiency change threshold value WGDS. This establishes that the efficiency of EGR cooler  8  has changed by more than a certain amount, whereby a defect of EGR cooler  8  may be detected. 
         [0042]    To implement an engine control system, a specification about the exhaust gas recirculation rate (EGR rate) is normally required. Since the EGR rate, however, depends heavily on the temperature of the recirculated exhaust gas, a specification about the current temperature T EGR  of the recirculated exhaust gas must be provided. The behavior of temperature detector  9 , however, is slow and hence not suited for this purpose. Example embodiments of the present invention therefore provide for the temperature T EGR  of the recirculated exhaust gas to be derived from the exhaust gas temperature and the efficiency of EGR cooler  8  at a certain mass flow. 
         [0043]    Since the efficiency η cooler  of EGR cooler  8  may change during the life of engine system  1 , it is sensible to adapt the cooler model for modeling the temperature T EGR  of the cooled recirculated exhaust gas. Starting from a conventional cooler model  20 , which ascertains a temperature specification for the temperature T EGR     —     model  of the recirculated cooled exhaust gas as a function of the exhaust gas temperature T 3 , the cooling water temperature T cooling water  and the efficiency η cooler     —     intact  of intact EGR cooler  8  (a reference cooler), and ascertains as a function of the efficiency change Δη cooler  or the debounced efficiency change Δη cooler     —     debounced  ascertained in efficiency change unit  10 . The temperature T EGR  of the cooled recirculated exhaust gas, which was used so far by the exhaust gas recirculation control system, may be adapted as a function of the efficiency change Δη cooler  or the debounced efficiency change Δη cooler     —     debounced , ascertained in efficiency change calculation unit  10 , as an efficiency change correction value Δη cooler     —     correction  and as a function of modeled temperature specification T EGR     —     model  as well as exhaust gas temperature T 3  and cooling water temperature T cooling water . 
         [0044]    In adaptation block  21 , the adapted temperature T EGR     —     adapted  behind EGR cooler  8  is ascertained as a function of the ascertained efficiency change Δη cooler     —     correction  according to the following formula: 
         [0000]        T   EGR     —     adapted   =T   EGR     —     model +Δη*( T   3   −T   cooling water ) 
         [0045]    In adaptation unit  21 , additional measures may be applied such as e.g. a maximum limitation of efficiency change Δη or the definition of a learning rate, as are sufficiently known for adaptation methods. 
         [0046]    The adaptation method shown in  FIG. 3  makes it possible to obtain a current value of recirculated cooled exhaust gas T EGR     —     adapted , which in contrast to the temperature specification obtained from temperature detectors  9  represents a current value of the temperature of the recirculated exhaust gas. A time delay of the temperature value due to the lag of temperature detector  9  may thus be avoided such that the exhaust gas recirculation control system is able to react more quickly to temperature changes in the recirculated exhaust gas so as to set the EGR rate to the desired value. 
         [0047]      FIG. 4  shows a schematic representation of the method according to example embodiments of the present invention for monitoring the EGR cooler. In comparison to the example embodiment of  FIG. 2 , enabling unit  11 , counter unit  12  and first comparator unit  13  are developed identically. 
         [0048]    An efficiency calculation unit  30  calculates an efficiency η cooler  of the cooler from the temperature specification of temperature detector T EGR , from exhaust gas temperature T 3  and from cooling water temperature T cooling water  according to the following formula: 
         [0000]    
       
         
           
             
               η 
               cooler 
             
             = 
             
               
                 
                   T 
                   3 
                 
                 - 
                 
                   T 
                   EGR 
                 
               
               
                 
                   T 
                   3 
                 
                 - 
                 
                   T 
                   
                     cooling 
                      
                     
                         
                     
                      
                     water 
                   
                 
               
             
           
         
       
     
         [0049]    Efficiency η cooler  is supplied to a filter unit  31 , which low-pass filters efficiency η cooler . Efficiency η cooler  is provided permanently. The filtered efficiency of the cooler η cooler     —     filtered , however, is only calculated if the enabling conditions obtain in accordance with enabling signal FS (see specific embodiment of  FIG. 2 ). Furthermore, filter unit  31  receives the cumulative total enabling time t cumulative  as the output of counter unit  12 . Total enabling time t cumulative  may then correspond to the filter time constant or be a function of it. This results in the formation of an average of the absolute efficiency. This increases the robustness of the calculation such that even changes in the dynamics of temperature detector  9 , e.g. by soot deposits, or different dynamics of temperature detector  9  and the engine model for calculating exhaust gas temperature T 3  only have a small influence on the result. The filtering of efficiency η cooler  is described by the following equation: 
         [0000]    
       
         
           
             
               η 
               cooler_filtered 
             
             = 
             
               
                 ∫ 
                 
                   
                     η 
                     cooler 
                   
                    
                   
                      
                     T 
                   
                 
               
               
                 t 
                 cumulative 
               
             
           
         
       
     
         [0050]    Efficiency η cooler  is integrated during the total enabling time t cumulative  and is divided by cumulative total enabling time t cumulative . 
         [0051]    In addition to the example embodiment of  FIG. 2 , in the example embodiment of  FIG. 4 , exhaust gas mass flow dm EGR  through recirculation line  6  is filtered in that it is integrated during total enabling time t cumulative  and is divided by total enabling time t cumulative  so as to form the average of the efficiency. The filtering occurs according to the following equation: 
         [0000]    
       
         
           
             
               d 
                
               
                   
               
                
               
                 m 
                 EGR_filtered 
               
             
             = 
             
               
                 ∫ 
                 
                   d 
                    
                   
                       
                   
                    
                   
                     m 
                     EGR 
                   
                    
                   
                      
                     T 
                   
                 
               
               
                 t 
                 cumulative 
               
             
           
         
       
     
         [0052]    Exhaust gas mass flow dm EGR  is filtered in mass flow filter  32 . If cumulative total enabling time t cumulative  has exceeded a total enabling time threshold value GZS (determined by first comparator unit  13 ), then filtered efficiency η cooler     —     filtered  of the cooler and exhaust gas mass flow dm EGR     —     filtered  are provided as debounced variables via debouncing units  14  and another debouncing unit  33  for debouncing exhaust gas mass flow dm EGR . This provides robust debounced variables for cooler efficiency η cooler     —     debounced  and an exhaust gas mass flow dm EGR     —     debounced  that is consistent with them. These are used in the adaptation model for adapting the cooler efficiency ηcooler. Furthermore, the comparison in the additional comparator unit  16  is able to determine an error if efficiency η cooler     —     debounced  of EGR cooler  8  falls below a certain efficiency threshold value WGS. 
         [0053]    In this exemplary embodiment, efficiency threshold value WGS is not constant, but is a function of mass flow dm EGR . Efficiency threshold value WGS is calculated from an efficiency threshold value offset WGSoff and a variable value that results from a characteristics map  34 . 
         [0054]      FIG. 5  shows characteristic curves of a cooler model for the efficiency η cooler  of EGR cooler  8  as a function of exhaust gas mass flow dm EGR  for an intact cooler (reference cooler) and efficiency η cooler bypass open  for an EGR cooler  8  that is suppressed entirely with the aid of a bypass. The characteristic curves are usually ascertained by measurement at an engine test stand for an internal combustion engine  2 . 
         [0055]    In the representation of  FIG. 5 , a debounced efficiency η cooler , which is calculated from the temperature specification of temperature detector  9  as previously described, is plotted in exemplary fashion against exhaust gas mass flow dm EGR  within the two characteristic curves for the intact cooler and the suppressed EGR cooler  8 . The plotted operating point η cooler     —     adaptation , dm EGR     —     adaptation  indicates the current cooler behavior or its performance. In order to be able to infer from this operating point the adapted characteristic curve of EGR cooler  8  as a function of exhaust gas mass flow dm EGR , the values for each value of the exhaust gas mass flow are interpolated between the characteristic curves for η cooler     —     intact  und η cooler     —     bypass     —     open  such that the dashed characteristic curve for η cooler     —     corrected  results. The interpolation may be performed, for example, in that for each value of exhaust gas mass flow dm EGR  the distance between the efficiencies for η cooler     —     intact  and η cooler     —     bypass     —     open  is divided into a ratio in which the correction value of the efficiency η cooler     —     adaptation  divides the distance between efficiencies for η cooler     —     intact  and η cooler     —     bypass     —     open  at point dm EGR     —     adaptation . The following applies: 
         [0000]    
       
         
           
             α 
             = 
             
               
                 ( 
                 
                   
                     
                       
                         
                           η 
                           cooler_adaptation 
                         
                         ( 
                         
                           
                             d 
                              
                             
                                 
                             
                              
                             
                               m 
                               EGR_adaptation 
                             
                           
                           - 
                         
                       
                     
                   
                   
                     
                       
                         
                           η 
                           
                             cooler_bypass 
                              
                             _open 
                           
                         
                          
                         
                           ( 
                           
                             d 
                              
                             
                                 
                             
                              
                             
                               m 
                               EGR_adaptation 
                             
                           
                           ) 
                         
                       
                     
                   
                 
                 ) 
               
               
                 ( 
                 
                   
                     
                       
                         
                           
                             η 
                             
                               cooler 
                               intact 
                             
                           
                            
                           
                             ( 
                             
                               d 
                                
                               
                                   
                               
                                
                               
                                 m 
                                 EGR_adaptation 
                               
                             
                             ) 
                           
                         
                         - 
                       
                     
                   
                   
                     
                       
                         
                           η 
                           
                             cooler_bypass 
                              
                             _open 
                           
                         
                          
                         
                           ( 
                           
                             d 
                              
                             
                                 
                             
                              
                             
                               m 
                               EGR_adaptation 
                             
                           
                           ) 
                         
                       
                     
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             
               
                 η 
                 cooler_corrected 
               
                
               
                 ( 
                 
                   d 
                    
                   
                       
                   
                    
                   
                     m 
                     EGR 
                   
                 
                 ) 
               
             
             = 
             
               α 
               · 
               
                 ( 
                 
                   
                     
                       
                         
                           
                             η 
                             cooler_intact 
                           
                            
                           
                             ( 
                             
                               d 
                                
                               
                                   
                               
                                
                               
                                 m 
                                 EGR 
                               
                             
                             ) 
                           
                         
                         - 
                       
                     
                   
                   
                     
                       
                         
                           η 
                           
                             cooler_bypass 
                              
                             _open 
                           
                         
                          
                         
                           ( 
                           
                             d 
                              
                             
                                 
                             
                              
                             
                               m 
                               EGR 
                             
                           
                           ) 
                         
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0056]    This interpolation may be performed for example in a control unit of internal combustion engine  3 . 
         [0057]    When ascertaining the corrected characteristic curve it is sensible to limit the adaptation. For example, the adaptation should be performed in limited adjustable step sizes in order to increase the robustness of the adaptation method. For example, if the currently valid characteristic curve for the corrected cooler efficiency η cooler     —     corrected  lies above the calculated operating point η cooler , dm EGR , then for example the characteristic curve (i.e. all values of the efficiencies η cooler ) may be adapted by a defined percentage of the absolute value of the efficiency or of the differential value of the efficiencies η cooler     —     intact  for the intact cooler and the efficiency η cooler bypass open  for the bypassed cooler. 
         [0058]    Efficiency threshold value WGS is now determined as the sum of constant efficiency threshold value offset WGSoff and an efficiency differential value Δη ascertained from a characteristics map  34 , which is provided as a function of exhaust gas mass flow dm EGR     —     debounced . The efficiency threshold value WGS dependent on exhaust gas mass flow dm EGR     —     debounced , makes it possible to increase the robustness of monitoring in that the dependent efficiency threshold value is increased in the direction of smaller EGR mass flows corresponding to efficiency characteristic curve η cooler     —     corrected . 
         [0059]      FIG. 5  schematically shows the adaptation of cooler efficiency η cooler  with the aid of the corrected efficiency characteristic curve, which is shown in exemplary fashion as the dashed efficiency characteristic curve in  FIG. 5 . The adaptation of temperature T EGR  is performed with the aid of corrected cooler efficiency η cooler     —     corrected , which results from the characteristic curve of  FIG. 5 , and the exhaust gas temperature T 3  and cooling water temperature T cooling  water. The following applies: 
         [0000]        T   EGR     —     adapted   =T   3 −η cooler     —     corrected ( T   3   −T   cooling water ) 
         [0060]    If the corrected efficiency characteristic curve is available, it may be used to calculate, in the air system model using the same algorithms as before, for an intact cooler, the adapted temperature T EGR     —     adapted  for the current real cooler state, which has very good dynamics as contrasted with the temperature specification of temperature detector  9  that is provided with a lag.