Abstract:
A method for monitoring the enabling of a system which is assigned to an internal combustion engine, in which the readiness for operation of the system is dependent on an operating temperature, is characterized in that a fault in the enabling of the system is inferred ( 106 ) if operation of the system cannot be detected on expiry of a predeterminable time period after the starting of the internal combustion engine or on the recurrence of readiness for operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for monitoring the enabling of a system which is assigned to an internal combustion engine, in which the readiness for operation of the system is dependent on an operating temperature. 
         [0002]    In order to reduce nitrogen oxides in the exhaust gas of an internal combustion engine, use is already being made of so-called SCR catalytic converters (Selective Catalytic Reduction) in the exhaust section of the internal combustion engine to reduce the nitrogen oxides contained in the exhaust gas to nitrogen in the presence of a reducing agent. Ammonia is required for the reaction to take place, and this is added to the exhaust gas. In general, an aqueous urea solution is used for this purpose, being injected into the exhaust line upstream of the SCR catalytic converter with the aid of a metering device. Ammonia is formed from this solution and acts as a reducing agent. 
         [0003]    For optimum aftertreatment of the exhaust gas, the reducing agent is injected into the exhaust line as required. The metered quantity is calculated in accordance with NOx emissions upstream of the SCR catalytic converter. This calculation includes the reducing agent pressure, which is adjusted to a predeterminable desired pressure. 
         [0004]    Since the metering system for the SCR catalytic converter is a component of significance for the exhaust gas, it is necessary to monitor the ability to function of the metering system. The current practice is to monitor the time up to the closing of the control loop (Time To Closed Loop—TTCL) of the SCR metering system or to the enabling of this system on the basis of an evaluation of the temperature of the SCR catalytic converter and the readiness for metering of the SCR system. During this process, a check is made to determine whether the temperature of the SCR catalytic converter is above an applicable threshold and whether the SCR system is ready for metering. If both conditions are satisfied and metering is not active after the expiry of an applicable time, a fault is set. The temperature of the SCR catalytic converter is calculated on the basis of a signal from a temperature sensor arranged upstream of the SCR catalytic converter. The signal from this temperature sensor is generally denoted by T6. In order to ensure that the information provided by the TTCL monitoring is reliable, it is necessary to monitor the functioning of the temperature sensor or the signal T6 from said sensor. Normally, the plausibility of T6 is checked by comparison with a model temperature, which is, in turn, calculated from the signal from another temperature sensor fitted in the exhaust system upstream of the temperature sensor mentioned. The signal from this further temperature sensor in the exhaust system is generally denoted by T5. 
         [0005]    Since temperature sensors are generally subject to tolerances, it is not possible to rule out a certain inaccuracy in the plausibility checking and monitoring. It is therefore the underlying object of the invention to provide improved monitoring of the enabling of a system assigned to the internal combustion engine, in particular for the operation of an SCR metering system. 
       SUMMARY OF THE INVENTION 
       [0006]    In the method according to the invention for enabling a system which is assigned to an internal combustion engine and the readiness for operation of which is dependent on an operating temperature, it is the case according to the invention that a fault in the enabling of the system is inferred if operation of the system cannot be detected on expiry of a predeterminable time period after the starting of the internal combustion engine. If there is an interruption in readiness for operation during the current cycle of the internal combustion engine, the invention envisages that a check is made to determine whether operation of the system can be detected after the expiry of a predeterminable time period from the discontinuation and recurrence of readiness for operation. Here, therefore, the starting of the internal combustion engine can be said to be equivalent to the recovery of readiness for operation after a discontinuation of readiness for operation. At the heart of the invention is that it is the absolute time, i.e. the total time expired, between the starting of the internal combustion engine or recurrence of readiness for operation and the beginning of operation of the component which is used as the diagnostic criterion in this approach to monitoring. Unlike known TTCL monitoring methods, evaluation is not based on a relative time from the exceeding of a temperature threshold, which relies on signals from a temperature sensor, e.g. T6. The method according to the invention is independent of temperature sensors fitted in the exhaust line and therefore the inaccuracies associated with temperature sensors are not relevant in the method according to the invention. The method according to the invention thus meets the requirements made, for example, by authorities more effectively than a conventional monitoring method. 
         [0007]    The expression “system” in the way it is used here is to be taken to mean systems or partial systems which are assigned to the internal combustion engine. These can be hardware components or software functions, for example. 
         [0008]    In a preferred embodiment of the method according to the invention, the predeterminable time period, after the expiry of which a fault is inferred, if appropriate, is determined by means of a first time model. The speed of the internal combustion engine and/or the fuel injection quantity are preferably included in the first time model. In particular, the first time model uses the engine speed and injection quantity as a basis for calculating the predeterminable time period, with the possibility of incorporating corrections by way of further factors, such as the RHU status (Rapid Heat Up), the temperature, e.g. the outside temperature, and/or the vehicle speed. RHU refers to an operating mode used especially in SCR systems with the aim of reaching the exhaust gas temperature required by the components of the exhaust gas aftertreatment system as quickly as possible. However, the use of RHU may not be appropriate under certain circumstances (e.g. warm starting, frozen reducing agent etc.). Since RHU has a major effect on the exhaust gas temperature and hence on the time to the beginning of metering, the time model is preferably corrected accordingly to enable these circumstances too to be covered by means of the monitoring according to the invention. In this case, the model can calculate a threshold value (tStart) for the time from the starting of the engine. If the engine running time reaches or exceeds the threshold value tStart calculated from the time model, an assessment is made to determine whether operation of the system, e.g. the enabling of metering by an SCR metering system, can be detected. This can be ascertained, in particular, from whether a closed control loop (Closed Loop—CL) has been achieved. If CL has not been achieved, a fault is set. 
         [0009]    In a particularly preferred embodiment of the method according to the invention, measures for increasing the operating temperature are initiated if operation of the system is not detected on expiry of said first predeterminable time period. After the expiry of a second predeterminable time period, within which the temperature-increasing measures are carried out, a check is made to determine whether operation of the system can be detected. If operation of the system cannot be detected after the expiry of the second predeterminable time period either, the fault is set. This initiation of at least one measure for increasing the operating temperature on suspicion of a fault, which is referred to as escalation, increases the reliability and robustness of the diagnostic method according to the invention. However, if a robust application is possible even without the escalation, escalation measures can be dispensed with. 
         [0010]    The second predeterminable time period is preferably determined by means of a second time model, which calculates a second threshold value (tEnd) for the time on the basis especially of the engine speed and the fuel injection quantity, it being possible for the calculation to include correction factors relating, for example, to the temperature (e.g. measurable or calculable outside temperature) and/or the speed of the motor vehicle. Once the time tEnd is reached or exceeded, a check is once again made to determine whether operation or the closed control loop (CL) of the system has been achieved. If CL has not been achieved, a fault is finally set. 
         [0011]    The temperature-increasing measures can be suspended or ended if operation (CL) of the system is achieved and/or if the engine running time reaches the threshold value tEnd calculated from the second time model. If the measures for increasing the temperature are suspended in accordance with the operating point, the second time model is reinitialized at the time when the measure(s) is (are) reactivated in order to enable the method according to the invention to be completed. 
         [0012]    A so-called ghost counter, i.e. a counter which, in asymmetric diagnoses of this kind, calculates the time that would be required to detect a fault, is preferably implemented in order to carry out the method according to the invention, simplifying the evaluation of the method according to the invention. The ghost counter can be incorporated into the second time model, in particular. To determine the frequency with which diagnoses are run (IUMPR), it is necessary to determine the time required to find a fault. Since, according to the invention, provision can be made for the possibility of detecting a fault only after the expiry of the second time model, but freedom from faults (OK) may already have been detected after the expiry of the first time model, the ghost counter must be used when the system is OK in order to determine the time that would be required to detect a fault. This can be implemented by calculating the second time model without escalation being active if the system is OK. 
         [0013]    As an alternative to the use of the time models, it is also possible to use a temperature model for T6. On the assumption that the attainment of a defined exhaust gas temperature is the limiting parameter for the achievement of the closed loop condition, time and temperature modeling are equivalent, given a predetermined driving profile and ambient conditions. 
         [0014]    The method according to the invention is suitable for monitoring the enabling of systems, the operation of which is based on a closed control loop. The method according to the invention can furthermore also be employed for monitoring the enabling of adaptive and/or open-loop control systems. 
         [0015]    In a preferred embodiment of the method according to the invention, the system which is assigned to the internal combustion engine is the metering system of an SCR catalytic converter. As described at the outset, the hitherto customary SCR-TTCL monitoring was based inter alia on evaluation of the temperature of the SCR catalytic converter, which is determined on the basis of signals (T6) from a temperature sensor arranged upstream of the SCR catalytic converter. The monitoring method according to the invention is independent of temperature sensors in the exhaust line and, as a result, the problems associated with temperature sensors and inaccuracies associated therewith in the monitoring and plausibility checking of the SCR metering system do not occur in the method according to the invention. The operation of the SCR metering system which is to be monitored as part of the method according to the invention can be determined by active metering of the metering system. From the active metering it is possible to ascertain whether SCR-CL has been achieved. The expiry of the predeterminable time periods can be determined, in particular, with the aid of a timer which begins to count when the engine is started. If closed-loop control is suspended during the current driving cycle, i.e. when a loss of the enabled status occurs, the timer begins to count upon recurrence of the enabled status, i.e. after the suspension of the enabled status. This case can occur, for example, when the temperature of the SCR catalytic converter falls during the driving cycle owing to the driving profile, e.g. overrun operation [Krfl], to such an extent that the temperature required for SCR metering is no longer available. 
         [0016]    In other embodiments of the method according to the invention, the system is a fuel mass observer (FMO) or an exhaust gas sensor, e.g. a lambda probe, an NOx sensor or a PM sensor. The fuel mass observer is a type of lambda closed-loop controller, the aim of which is to compensate for a drift in the injection and air system. Enabling of the lambda probe is required for closed-loop operation. 
         [0017]    The invention furthermore comprises a computer program which carries out all the steps of the method according to the invention when it is carried out on a computing device or a control unit. Finally, the invention comprises a computer program product having a program code, which is stored on a machine-readable medium, for carrying out the method described when the program is carried out on a computing device or a control unit. Implementing the method according to the invention as a computer program or computer program product has the advantage that, by installing the computer program in the control unit of an internal combustion engine, for example, or in some other control unit on the motor vehicle, the advantages of the method according to the invention can be readily employed during the operation of the internal combustion engine without the need to install additional components. Thus, the method according to the invention can also be used to particular advantage on already existing motor vehicles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Further features and advantages of the invention will emerge from the following description of embodiments in conjunction with the drawings. In this context, the individual features can each be implemented singly or in combination. 
           [0019]    In the drawings: 
           [0020]      FIG. 1  shows a first flow chart relating to the execution of a preferred embodiment of the method according to the invention; 
           [0021]      FIG. 2  shows another flow chart relating to the execution of another preferred embodiment of the method according to the invention, and 
           [0022]      FIG. 3  shows another flow chart relating to the execution of another preferred embodiment of the method according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows a flow chart intended to illustrate one embodiment of the method according to the invention. After the starting  101  of the internal combustion engine, the timer is started in step  102  in order to record the time. In parallel with this, in step  103 , the first time model relating to the attainment of the threshold value tStart is started. In step  104 , a check is made to determine whether the threshold value tStart calculated by means of the first time model has been reached by the engine running time. If this is not the case, the program loops back to the start of step  104 . If this is the case, a check is made, in step  105 , to determine whether operation of the system can be detected, i.e. whether a closed control loop (CL), for example, of the SCR metering system, for example, can be detected. If operation cannot be detected, a fault is set (error bit  0 →1) and a test bit is set to 1 (test bit  0 →1) in the output  106 . If, in step  105 , the program ascertains that operation of the system can be detected, the output given in step  107  is that no fault can be detected (error bit  0 →0) and the test bit is likewise set to 1 (test bit  0 →1). 
         [0024]      FIG. 2  shows another flow chart intended to illustrate the method according to the invention and, in this embodiment of the method, an escalation, i.e. at least one measure for increasing the operating temperature, is initiated on suspicion of a fault, and a fault is set only if operation of the system cannot be detected even after the temperature-increasing measures. After the starting of the internal combustion engine in step  201 , the timer is first of all started in step  202 . In parallel with this, the first time model is started in step  203  in order to calculate the threshold value tStart. In step  204 , a check is made to determine whether the threshold value tStart calculated by means of the first time model has been reached by the engine running time. If this is not the case, the program loops back to the start of step  204 . If the inquiry in step  204  obtains an affirmative answer, a check is made in step  205  to determine whether operation of the system can be detected, e.g. whether a closed control loop (CL) of an SCR metering system has been achieved. If this is the case, the output from step  206  is that operation is detectable, that is to say, for example, that CL has been achieved (CL status  0 →1). If, in step  205 , the program determines that operation cannot be detected, a corresponding output is issued in step  207  (CL status  0 →0). In this case, the temperature-increasing measures are started in step  208 . At the same time, the second time model for calculating the threshold value tEnd and, in parallel, a timer are started (step  209 ). In step  210 , the program inquires whether operation can be detected or, in the case of the SCR metering system as an example, whether CL has been achieved. If this is the case, escalation is ended in step  211 , and the output from step  212  is that CL has been achieved (CL status  0 →1). If the check in step  210  shows that CL has not been achieved, a check is made in step  213  to determine whether the threshold value tEnd from the second time model has been reached. If this is the case, the output from step  215  is that CL has not been achieved (CL status  0 →0). Where appropriate, the escalation measures are ended before this in step  214 . In step  216 , a check is made to determine whether CL has been achieved. If this is not the case, a fault is set (error bit  0 →1) in the output  217 , and an indication is given that diagnosis has been carried out (test bit  0 →1). If, in step  216 , the program detects that CL has been achieved, the output  218  is issued that no fault is present (error bit  0 →0) and that diagnosis has been run (test bit  0 →1). On the basis of the output  217  or  218 , diagnosis is ended (step  219 ). 
         [0025]      FIG. 3  shows another flow chart intended to illustrate another embodiment of the method according to the invention. In this case, monitoring is carried out in a manner comparable to the method illustrated in  FIG. 2  with temperature-increasing measures (escalation) on suspicion of a fault. Furthermore, a so-called ghost counter is used in parallel with the escalation in order, when necessary, to delay diagnosis sufficiently to ensure that the time that would be required to detect a fault expires. In other words, in the OK condition the second time model is calculated without escalation being active. After the starting  301  of the internal combustion engine or motor, the timer for the engine running time is started in step  302 . At the same time, the first time model for calculating the threshold value tStart is started in step  303 . In step  304 , a check is made to determine whether the threshold value tStart calculated by means of the first time model has been reached by the engine running time. If this is not the case, the program loops back to the start of step  304 . If the inquiry in step  304  obtains an affirmative answer, a check is made in step  305  to determine whether operation of the system can be detected, that is to say, for example, whether a closed control loop (CL) of the SCR metering system has been achieved. Where the metering system for an SCR catalytic converter is used as an example, CL (SCR-CL) can be ascertained from whether metering has been activated or demanded. If this is the case, the output from step  306  is that the CL status is OK (SCR-CL status  0 →1). If this is not the case, a check is made in step  307  to determine whether the operating conditions for the initiation of temperature-increasing measures have been satisfied. If this is not the case, the program loops back to the start of step  307 . If this is the case, so-called escalation is started in step  308 , i.e. at least one measure for increasing the operating temperature, e.g. the temperature of the SCR catalytic converter, is started. Examples of temperature-increasing measures are retardation of the main injection, a late post-injection or throttling. In parallel with this, the second time model for calculating the threshold value tEnd is started (step  309 ). The calculation of the threshold values tStart and tEnd includes, in particular, the speed of the internal combustion engine and the fuel injection quantity, and corrections relating to the temperature and/or the speed of the motor vehicle driven by the internal combustion engine can be allowed for in a manner known per se. In step  310 , the program inquires whether suspension of escalation is required due to operation. If this is the case, the program loops back to the start of step  307 . If this is not the case, a check is made in step  311  to determine whether operation of the system can be detected, e.g. whether a closed control loop CL, SCR-CL for example, has been achieved. If this is the case, escalation is ended in step  312 , i.e. the temperature-increasing measures are stopped. The output from step  313  is then that the CL status is OK (SCR-CL status  0 →1). If operation, in particular a closed control loop (CL), cannot be detected in step  311 , a check is made in step  314  to determine whether the threshold value tEnd determined by means of the second time model in step  309  has been reached. This inquiry is also made after escalation is stopped in step  312 . If this is the case, the temperature-increasing measures are, if appropriate, ended in step  315 , and the output in step  316  is that the CL status is not OK (SCR-CL status  0 →0). Following on from step  315 , another inquiry is made in step  317  to determine whether the CL status is OK, i.e. whether operation of the system can be detected. If this is the case, the output in step  318  is that the monitoring method has been carried out (test bit  0 →1) and no fault is present (error bit  0 →0). If the inquiry  317  shows that the CL status is not present, a fault is output in step  319  (error bit  0 →1) on completion of diagnosis (test bit  0 →1). If the inquiry in step  314  shows that the threshold value tEnd has not been reached, the program loops back to the start of step  310 . 
         [0026]    If, in step  305 , the program ascertains that operation of the system can be detected (SCR-CL status  0 →1), diagnosis is delayed to such an extent, by implementing the so-called ghost counter, that the time required to run through the diagnosis of an OK system corresponds to that of a faulty system. For this purpose, a check is made in step  320  to determine whether the operating conditions for temperature-increasing measures have been satisfied. If this is not the case, the program loops back to the start of step  320 . If this is the case, the second time model is started in step  321 . A check is then made in step  322  to determine whether suspension of the temperature-increasing measures has taken place due to operation. If the inquiry in step  322  shows that suspension of the temperature-increasing measures has taken place due to operation, the program loops back to the start of step  320 . If the inquiry  322  shows that suspension of escalation due to operation has not taken place, a check is made in step  323  to determine whether the threshold value tEnd from the second time model has been reached. If this is not the case, the program loops back to the start of step  322 . If this is the case, step  317  is carried out and the program inquires whether the CL status is OK, i.e. whether operation of the system can be detected. If this is the case, the output  318  is that there is no fault after the monitoring method has been completed. If this is not the case, the output  319  is that the CL status is not OK, and a fault is set. 
         [0027]    The ghost counter (steps  320  to  323 ) compensates for the time difference between an OK system and a defective system during the execution of diagnosis, thus allowing a reliable numerator to be incremented in order to enable diagnosis execution frequency to be determined.