Abstract:
A method for operating a metering valve and a device for performing the method, providing for diagnosis of the metering valve which defines a flow rate of a reagent to be introduced into an exhaust gas area of an internal combustion engine. The diagnosis is performed on the basis of an analysis of a measure for the flow rate during a diagnosis time. According to a first embodiment, after a diagnosis start signal has occurred with the metering valve closed, the reagent is brought to a predefined diagnosis starting pressure via a pump; the metering valve is then set at a predefined flow rate and the pressure difference occurring during the diagnosis time is analyzed. According to another embodiment, the amount of reagent delivered by the metering valve during the diagnosis time into a graduated beaker is analyzed.

Description:
BACKGROUND INFORMATION 
     German Patent Application No. DE 101 39 142 describes an exhaust gas aftertreatment unit for an internal combustion engine in which the concentration of a urea-water solution in a tank is determined to permit accurate metering of the urea-water solution into the exhaust gas area of the internal combustion engine. The urea-water solution is a reagent that acts as a reducing agent in an SCR catalytic converter, where it reduces the nitrogen oxides contained in the exhaust of the internal combustion engine. In a first reaction step, the urea present in the urea-water solution reacts with water (undergoing hydrolysis) to form ammonia and carbon dioxide. In a second reaction step, NO and NO 2  react with ammonia to form nitrogen and water. The flow rate of the urea-water solution is adjusted by a metering valve and must not exceed an upper limit or drop below a lower limit. If it drops below the lower limit, the SCR catalytic converter is ineffective, and if it exceeds the upper limit, a breakthrough of ammonia occurs. 
     An object of the present invention is to provide a method for operating a metering valve and a device for performing the method, making it possible to maintain a specified flow rate of a reagent to be introduced into an exhaust gas area of an internal combustion engine. 
     SUMMARY OF THE INVENTION 
     In a procedure according to the present invention, a measure of the flow rate of a reagent through the metering valve is analyzed during a measurement time as part of a diagnosis. The procedure according to the present invention ensures the accuracy of the metered addition of reagent into the exhaust gas area of an internal combustion engine over the entire service life of the metering valve. The diagnosis thus contributes toward compliance with the exhaust limits during the entire operating period of the internal combustion engine. 
     According to a first embodiment, the diagnosis is triggered by a start signal supplied by a diagnosis device. The first embodiment is suitable for performing the diagnosis as part of an inspection of the internal combustion engine which may be performed at a service shop, for example. The measure of the flow rate is determined during a specified measurement time during which the reagent is collected in a graduated beaker. On the basis of a comparison with a reference value which is determined when the metering value is new, for example, and stored in a memory of a control unit, it is possible to decide whether it is sufficient to merely take into account an adjustment value or if the metering valve must be replaced. 
     According to another embodiment, a pressure difference is used as the measure of the flow rate through the metering valve. With this measure, it is possible to perform a diagnosis during downtime and also during operation of the internal combustion engine, even without time spent in a service shop. According to one embodiment of the diagnosis test, after a diagnosis start signal has occurred, the reagent is brought to a predefined starting pressure by a pump while the metering valve is closed; the metering valve is then set at a predefined flow rate and the pressure difference occurring due to the pressure drop during the measurement time is analyzed. 
     According to an embodiment of this method, the pressure difference is a fixedly predefined level, and a warning signal is supplied when the measurement time exceeds a predefined diagnosis time limiting value. According to an alternative embodiment, the diagnosis time is preset at a fixed value and a warning signal is delivered if the pressure difference exceeds a predefined pressure difference limiting value. 
     The procedure according to the present invention may be used to adapt a diagnosis signal delivered by a metering control unit on the metering valve as a function of the diagnosis result. Wear on the metering valve may be compensated within certain limits through this measure, so that replacement of the metering valve may be postponed. 
     The diagnosis may be initiated, for example, by a diagnosis start signal supplied by an internal combustion engine control unit. An after-running control unit is advantageously provided, supplying a diagnosis start signal after the internal combustion engine has been turned off, so that the diagnosis may take place independently of operation of the internal combustion engine. The diagnosis start signal may also be supplied in particular by a freeze cycle counter that counts the number of freeze cycles of the system, in particular of the metering valve. The diagnosis may then be performed in particular after freezing of the reagent, which is critical for the metering valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an internal combustion engine within whose environment a method according to the present invention is performed for diagnosing a metering valve. 
         FIG. 2  shows a flow chart of a method, and  FIG. 3  shows a pressure curve over time. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an internal combustion engine  10  having an air sensor  12  in its intake area  11 , a metering device  14  and an emission control unit  15  in the exhaust gas area  13 . Air sensor  12  delivers an air sensor signal  17  to an internal combustion engine control unit  16 . Internal combustion engine control unit  16  also receives a rotational speed signal  18  supplied by internal combustion engine  10  and a setpoint signal  19 . 
     Internal combustion engine control unit  16  delivers a fuel signal  20  to internal combustion engine  10 , a metering signal  22  to a metering control unit  21  and a first diagnosis start signal  24  to a diagnosis control unit  23 . 
     Metering control unit  21  supplies a metering valve signal  26  to a metering valve triggering unit  25  and a pump signal  28  to a pump  27 . Metering control unit  21  also receives a diagnosis signal  29  and a connection signal  30  from diagnosis control unit  23 . 
     Metering valve triggering unit  25  is assigned to a metering valve  31 , which is connected to both metering device  14  and pump  27 . A temperature sensor  32  provided for metering valve  31  delivers a temperature signal  33  to a freeze cycle counter  34 . Freeze cycle counter  34  delivers a second diagnosis start signal  35  to diagnosis control unit  23 . 
     Pump  27  is connected to a reagent tank  36 . A pressure sensor  37  assigned to pump  27  delivers a pressure signal  38  to a signal analyzer  39 . 
     Signal analyzer  39  is contained in diagnosis control unit  23 . Diagnosis control unit  23  also receives a third diagnosis start signal  40  from an after-running control unit  60  and a fourth diagnosis start signal  42  from a diagnosis device  41 . 
     Signal analyzer  39  delivers a warning signal  43  to a signal device  44 . Signal analyzer  39  receives a diagnosis start pressure P 1 , a pressure difference limiting value P 3 max and a diagnosis time limiting value T 3 max. A timer  45  receives a timer start signal  46  from signal analyzer  39  and delivers a time signal  47  to signal analyzer  39 . 
       FIG. 2  shows a flow chart of the method according to the present invention. After a start  50 , metering valve  31  is closed in a first function block  51 . In a second function block  52 , pump  27  is turned on. A first query  53  determines whether diagnosis start pressure P 1  has been reached. If this is the case, then in a third function block  54 , pump  27  is turned off. Accordingly, metering valve  27  is opened with a predefined cross section in a fourth function block  55 . A second query  56  determines either whether a diagnosis time T has exceeded diagnosis time limiting value T 3 max or whether a pressure P has exceeded pressure difference limit P 3 max. If this is the case, warning signal  43  is then supplied in a fifth function block  57 . End  58  of the diagnosis is then reached. 
       FIG. 3  shows a curve for pressure P as a function of time T. Pressure P increases in a time range before diagnosis starting time T 1  until reaching diagnosis starting pressure P 1  at diagnosis starting time T 1 . During a diagnosis time T 3 , pressure P drops to a diagnosis end pressure P 2 . A pressure difference P 3  occurs between diagnosis starting pressure P 1  and diagnosis end pressure P 2 . 
     The method according to the present invention operates as described below. 
     Emission control unit  15  which is situated in exhaust gas area  13  of internal combustion engine  10  reduces at least one exhaust component such as soot or nitrogen oxides. Emission control unit  15  may therefore be designed, for example, as a filter or as a catalytic converter. It is assumed below that emission control unit  15  is provided for reducing nitrogen oxides and is designed as an SCR (selective catalytic reaction) catalytic converter. In the SCR catalytic converter known from the related art, a urea-water solution, which is stored in reagent tank  36 , is needed as the reagent. 
     The urea-water solution is a reagent which acts as a reducing agent in an SCR catalytic converter for the nitrogen oxides contained in the exhaust gas of the internal combustion engine. In a first reaction step, the urea contained in the urea-water solution is reacted (hydrolyzed) with water to form ammonia and carbon dioxide and in a second reaction step, NO and NO 2  are finally reacted with ammonia to form nitrogen and water. The concentration of the urea-water solution in the exhaust gas must not exceed an upper limit or drop below a lower limit. If it drops below the lower limit, the SCR catalytic converter is ineffective, and if it exceeds the upper limit, there is a breakthrough of ammonia. 
     To adjust the flow rate, i.e., flow quantity per unit of time, pump  27  and metering valve  31  are provided. Pump  27  brings the urea-water solution to a predefined pressure and metering valve  31  is adjusted by metering valve triggering unit  25  at a predefined flow cross section. 
     The flow rate to be preselected is a function of the concentration of nitrogen oxides and the exhaust mass flow in exhaust gas area  13  of internal combustion engine  10 . Internal combustion engine control unit  16  may estimate these values on the basis of air sensor signal  17  and/or fuel signal  20 , for example. If necessary, rotational speed signal  18  may also be taken into account. In addition, setpoint signal  19  which represents an intended torque may also be included. Internal combustion engine control unit  16  specifies metering signal  22  to be delivered to metering control unit  21 . Metering control unit  21  determines metering valve signal  26  which provides metering valve triggering unit  25  with information regarding the extent to which metering valve  31  is to be opened. Metering control unit  21  also controls pump  27  via pump signal  28 . 
     Metering valve  31  is subject to wear due to aging. Metering valve  31  may be exposed to mechanical stresses which occur in particular in freezing and/or thawing of the reagent. If a urea-water solution is used as the reagent, the freezing point is approximately −11° C. Therefore, a diagnosis which is performed by diagnosis control unit  23  is provided for checking on metering valve  31 . 
     The diagnosis may be triggered by internal combustion engine control unit  16  via first diagnosis start signal  24 . First diagnosis start signal  24  may be supplied, for example, in an operating state in which internal combustion engine  10  generates nitrogen oxide in small amounts, e.g., when idling. 
     According to a particularly advantageous measure, the number of freeze cycles of metering valve  31  is detected by freeze cycle counter  34 , which prompts a diagnosis via second diagnosis start signal  35 , either after each freezing or after a preselected number of freeze events. Freeze cycle counter  34  compares the temperature of metering valve  31  detected by temperature sensor  32  with a preselected threshold value which corresponds to the freezing point of the reagent. 
     After-running control unit  60 , which is still active after internal combustion engine  10  has been turned off, may prompt the diagnosis via third diagnosis start signal  40 . With this measure it is possible to perform the diagnosis without influence by the exhaust gas of internal combustion engine  10 . After-running control unit  60  is preferably contained in internal combustion engine control unit  16 . 
     The diagnosis of metering valve  31  may also be provided as part of a repair visit to the service shop. The diagnosis may be triggered by an operator using diagnosis device  41 , which delivers fourth diagnosis start signal  42  to diagnosis control unit  23 . 
     The diagnosis procedure will now be explained with reference to the flow chart illustrated in  FIG. 2  and the curve of pressure P as a function of time T as shown in  FIG. 3 . 
     Start  50  is reached by the occurrence of the first, second, third and/or fourth diagnosis start signals  24 ,  35 ,  40 ,  42 . Metering valve  31  is closed in first function block  51 . Diagnosis control unit  23  prompts metering valve  31  to be closed via diagnosis signal  29  which is sent to metering control unit  21 . 
     Pump  27  is turned on in subsequent second function block  52 . This procedure is also triggered by the occurrence of diagnosis signal  29 . Diagnosis starting time T 1  shown in  FIG. 1  is reached when it is found in first query  53  that pressure P has reached diagnosis starting pressure P 1 . Diagnosis starting pressure P 1  is sent to signal analyzer  39  as a preselected threshold value. Pressure sensor  37  detects that diagnosis starting pressure P 1  has been reached and then delivers pressure signal  38  to signal analyzer  39 . When diagnosis starting pressure P 1  is reached, pump  27  is turned off in subsequent third function block  54  and metering valve  31  is opened over a predefined cross section in following fourth function block  55 . 
     After opening metering valve  31 , a drop in pressure occurs in diagnosis time T 3  between diagnosis starting time T 1  and diagnosis end time T 2 , this pressure drop being given by pressure difference P 3 . To determine diagnosis time T 3 , timer  45  is provided and is started at diagnosis starting time T 1  via timer start signal  46  from signal analyzer  39  on reaching diagnosis starting pressure P 1 . Timer  45  sends diagnosis time T 3  back to signal analyzer  39  with time signal  47 . 
     Pressure difference P 3  may be used as a measure of the flow rate. The advantage of this measure is that it does not require any intervention in the device. Signal analyzer  39  may perform the diagnosis in two ways. According to a first exemplary embodiment, pressure difference P 3  may be fixedly predefined and diagnosis time T 3  may be compared with preselected diagnosis time limiting value T 3 max. According to another exemplary embodiment, diagnosis time T 3  may be fixedly predefined and pressure difference P 3  compared with preselected pressure difference limit P 3 max. The comparisons are performed in second query  56 . If one or the other limit T 3 max, P 3 max has not been exceeded, the procedure jumps directly to diagnosis end  58 . If a limiting value T 3 max, P 3 max has been exceeded, the procedure jumps to fifth function block  57  in which output of warning signal  43  is triggered. Warning signal  43  prompts signal unit  44  to release an acoustic and/or optical signal, for example, indicating to an operator that a service facility should be visited to test metering valve  31  and replace it, if necessary. 
     According to an expedient embodiment, correction signal  30  is delivered to metering control unit  21  as a function of the diagnosis result. Correction signal  30  allows compensation of deviations found in the flow rate of metering valve  31  which are within the tolerance before reaching limiting value T 3 max, P 3 max. In ascertaining metering valve signal  26  as a function of metering signal  22 , metering control unit  21  may also take into account correction signal  30  and correct the triggering of metering valve  31  adaptively. 
     The diagnosis may also be made volumetrically during a visit to a service shop. In this case, the diagnosis is triggered by diagnosis device  41  with fourth diagnosis start signal  42 . The quantity flowing through metering valve  31  during diagnosis time T 3  is collected in a graduated beaker. By comparing the collected amount with a reference value, a change, if any, may be ascertained. The reference value may be ascertained and stored in a memory of internal combustion engine control unit  16 , for example, when metering valve  31  is new. In this exemplary embodiment, correction signal  30  may be supplied by a manual intervention measure for adaptation of the metering quantity of reagent within preselected limits when there are deviations in the setpoint from the actual value. If the deviation between setpoint and actual value exceeds a predefined limit, metering valve  31  may have to be replaced, if necessary.