Abstract:
A method for operating an internal combustion engine, in whose exhaust gas system an exhaust gas treatment device is positioned, a reagent, which is possibly mixed with compressed air, being introduced into the exhaust gas system upstream from the exhaust gas treatment device, and a device for performing the method. The reagent is brought to a predefined reagent setpoint pressure using a reagent pump and subsequently metered using a reagent dosing valve. The reagent pressure existing upstream from the reagent dosing valve is detected. In the framework of a diagnosis, a time curve of the reagent setpoint pressure is predefined. The reagent pressure detected during the diagnosis is compared to at least one predefined threshold value. If the threshold value is exceeded, an error signal is provided. The corresponding diagnosis of a compressed air path may be provided alternatively or additionally to the diagnosis of the reagent path.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for operating an internal combustion engine, in whose exhaust gas system an exhaust gas treatment device is positioned, and in which a reagent is introduced into the exhaust gas system upstream from the exhaust gas treatment device, possibly mixed with compressed air, and a device for executing the method.  
       BACKGROUND INFORMATION  
       [0002]     An exhaust gas treatment device of an internal combustion engine, in which an SCR catalytic converter (selective catalytic reduction), which reduces the nitrogen oxides contained in the exhaust gas to nitrogen using the reducing agent ammonia, is used to reduce the NOx emissions, is described in German Patent Application No. DE 101 39 142. The ammonia is obtained from a urea-water solution in a hydrolysis catalytic converter positioned upstream from the SCR catalytic converter. The hydrolysis catalytic converter converts the urea contained in the urea-water solution with water into ammonia and carbon dioxide. To ensure exact dosing, the concentration of the urea-water solution is ascertained. The urea-water solution is brought to a predefined pressure using a pump. A dosing valve-fixes a predefined flow rate. Compressed air is mixed with the urea-water solution in a mixing chamber. The urea-water solution is sprayed together with the added air into the exhaust gas of the internal combustion engine in such a way that a largely uniform flow against the SCR catalytic converter is achieved.  
         [0003]     A method and a device, in which a pressurized reagent is also sprayed into the exhaust gas of an internal combustion engine upstream from an SCR catalytic converter, are described in German Patent Application No. DE 10 2004 018 221 (not previously published). The reagent pressure is fixed at a predefined reagent setpoint pressure as a function of a characteristic quantity. An operating variable of the internal combustion engine and/or a characteristic quantity of the exhaust gas of the internal combustion engine may be used as the characteristic quantity. The predefined reagent pressure setpoint value is regulated during a regulation in which the reagent actual pressure is detected by a reagent pressure sensor. Compressed air may be added to the reagent. The compressed air pressure may also be regulated to a predefined compressed air pressure setpoint value as a function of a characteristic quantity during a regulation, the compressed air actual pressure being detected by a compressed air pressure sensor. A defect of at least one of the pressure sensors may result in reduced performance capability of the SCR catalytic converter, with the consequence that unpurified exhaust gas may reach the environment.  
         [0004]     A method and a device, in which a pressurized reagent is sprayed into the exhaust gas of an internal combustion engine upstream from an SCR catalytic converter, are described in German Patent Application No. DE 10 2004 044 506 (not previously published). The compressed air is guided via a check valve which has an opening pressure. It provides for diagnostics of the compressed air pressure beginning at a starting instant with the closing of a compressed air regulating valve. A check is performed at at least a second instant to determine whether the compressed air pressure corresponds at least to a lower threshold value, which at least approximately corresponds to the opening pressure of the check valve added to the ambient air pressure. An error signal is provided when the condition is not fulfilled.  
         [0005]     A further method and a device, in which a pressurized reagent is sprayed into the exhaust gas of an internal combustion engine upstream from an SCR catalytic converter, are described in German Patent Application No. DE 101 59 849. Fuel which acts as a reducing agent for the NO 2  component in the exhaust gas in particular is provided as the reagent.  
         [0006]     The present invention is based on the object of providing a method for operating an exhaust gas treatment device of an internal combustion engine, in whose exhaust gas system an exhaust gas treatment device is positioned, and in which a reagent, which is possibly admixed with compressed air, is introduced into the exhaust gas system upstream from the exhaust gas treatment device, and a device for executing the method, which ensure high reliability.  
       SUMMARY OF THE INVENTION  
       [0007]     The method according to the present invention for operating an internal combustion engine first provides that a reagent is introduced into the exhaust gas system upstream from an exhaust gas treatment device. A reagent pump brings the reagent to a predefined reagent setpoint pressure. The reagent is dosed using a reagent dosing valve. The reagent pressure is detected upstream from the reagent dosing valve. During diagnostics, in which the reagent setpoint pressure has a predefined time curve, the detected reagent pressure is compared to at least one threshold value in at least one predefined state of the reagent dosing valve. If the threshold is exceeded, an error signal is provided.  
         [0008]     The method according to the present invention is alternatively or additionally suitable for diagnosing a compressed air path. The compressed air path contains a compressed air pump and a compressed air valve and provides for the detection of the compressed air pressure.  
         [0009]     The method according to the present invention significantly increases the reliability during operation of the internal combustion engine through the evaluation of the reagent and/or compressed air pressure, in particular if the reagent, such as fuel, is easily flammable. A leak in the reagent and/or compressed air path may be discovered using the method according to the present invention. Furthermore, it is possible to check at least one valve positioned in the reagent and/or compressed air path, which may stick in the closed or at least partially open state in case of fault. The method according to the present invention ensures that the exhaust gas treatment device is ready for use on the basis of the diagnostics and that a required exhaust gas purification is maintained.  
         [0010]     According to one embodiment, a periodic time curve of the reagent setpoint pressure and/or of the compressed air setpoint pressure is predefined. This measure makes it possible to fix the at least one threshold value and/or threshold value curve easily. In normal dosing operation, in which the reagent dosing valve and/or the compressed air valve is at least partially open, it may be checked whether the predefined time curve is reflected in the reagent pressure and/or compressed air pressure. For example, predefining a sinusoidal time curve of the reagent setpoint pressure and/or the compressed air setpoint pressure is suitable. Furthermore, predefining a pulsed time curve of the reagent setpoint pressure and/or the compressed air setpoint pressure is suitable.  
         [0011]     According to one embodiment, an operating state of the reagent dosing valve, in which the valve is closed, is predefined, and the threshold value is adjusted to the maximum predefined reagent setpoint pressure. Using this measure, it may be checked whether there is a leak in the reagent path or whether the reagent dosing valve sticks in the at least partially open state. A possibly occurring pressure drop may be established by ascertaining the pressure gradient or at least one pressure differential quotient and a subsequent comparison to at least one threshold value.  
         [0012]     According to one embodiment, an operating state of the reagent dosing valve, in which the valve is at least partially open, is predefined, and the threshold value is adjusted to the expected time curve. Using this measure, it may be checked whether the reagent dosing valve sticks in the closed state.  
         [0013]     Corresponding embodiments are possible in the compressed air path if the compressed air valve is electrically controllable. Otherwise, the threshold value is adjusted to the maximum compressed air setpoint pressure. Therefore, it may at least be determined whether the compressed air valve sticks in the closed state. Furthermore, according to one embodiment, the threshold value may be adjusted to the expected time curve of the predefined compressed air setpoint pressure. Therefore, it may also be determined whether the compressed air valve sticks in the closed state.  
         [0014]     One embodiment provides for the checking of a reagent pump and/or a compressed air pump.  
         [0015]     The device according to the present invention for operating an internal combustion engine primarily relates to a control unit which is implemented to perform the method.  
         [0016]     In particular, the control unit contains a diagnostic controller, a threshold value default for predefining the time curve of the setpoint pressure, and a comparator, which compares the at least one threshold value to the detected pressure.  
         [0017]     The control unit preferably contains at least one electrical memory, in which the method steps are stored as a computer program. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  shows a technical environment in which a method according to the present invention is executed.  
         [0019]      FIGS. 2   a ,  2   b  and  2   c  show signal curves as a function of time. 
     
    
     DETAILED DESCRIPTION  
       [0020]      FIG. 1  shows an internal combustion engine  10 , in whose intake area  11  an intake air detector  12  is positioned and in whose exhaust gas system  13  a reagent introduction device  14  and an exhaust gas treatment device  15  are positioned.  
         [0021]     Intake air detector  12  outputs an air signal msL to a control unit  20  and internal combustion engine  10  outputs a speed n to control unit  20 .  
         [0022]     Control unit  20  provides a fuel signal mK to a fuel metering device  30 .  
         [0023]     In a reagent path  31 , a reagent stored in a reagent tank  32  is brought to a predefined reagent setpoint pressure pReaSW by a reagent pump  33 . Reagent pump  33  is activated by a reagent pump activation signal  34 , which is provided by a reagent pump activator  35  positioned in control unit  20 .  
         [0024]     The reagent reaches a reagent dosing valve  36 , which is connected to a mixing chamber  37 . Furthermore, mixing chamber  37  is connected to a compressed air path  40 . After mixing chamber  37 , the reagent reaches reagent introduction device  14 .  
         [0025]     A reagent pressure sensor  41 , which provides reagent pressure pRea to both reagent pump activator  35  and also a comparator  42 , is positioned between reagent pump  33  and reagent dosing valve  36 . Comparator  42  provides an error signal F.  
         [0026]     A setpoint default  43  provides a reagent setpoint pressure pReaSw to both reagent pump activator  35  and also comparator  42 .  
         [0027]     A dosing valve activation signal msRea, provided by a reagent dosing controller  44 , to which a dosing signal  45  is supplied, is applied to reagent dosing valve  36 .  
         [0028]     A diagnostic controller  46  outputs a setpoint default signal  47  to setpoint default  43 , a comparison signal  48  to comparator  42 , and a diagnostic signal  49  to reagent dosing controller  44 .  
         [0029]     Compressed air path  40  contains a compressed air pump  50  to which a compressed air pump activation signal  51  is applied, a compressed air pressure sensor  52 , which provides a compressed air pressure pDI, and a compressed air valve  53 , which is positioned on mixing chamber  37 .  
         [0030]      FIG. 2   a  shows a predefined reagent setpoint pressure pReaSW as a function of time t. Between first and second instants t 1 , t 2 , a reagent nominal pressure pReaN is predefined as reagent setpoint pressure pReaSW. A time curve  60  of reagent setpoint pressure pReaSW is predefined at second instant t 2 . Time curve  60  has a maximum pReaMax at a third instant t 3 , a relative zero crossing  61  at a fourth instant t 4 , and a minimum pReaMin at a fifth instant t 5 . Minimum pReaMin is above an exhaust gas pressure pabg.  
         [0031]      FIG. 2   b  shows reagent pressure pRea as a function of time t. Between first and second instants t 1 , t 2 , reagent pressure pRea is predefined reagent nominal pressure pReaN. Between second and third instants t 2 , t 3 , reagent pressure pRea rises to maximum pReaMax of predefined reagent setpoint pressure pReaSW.  
         [0032]      FIG. 2   c  also shows reagent pressure pRea as a function of time t. Reagent pressure pRea is predefined reagent nominal pressure pReaN between first and second instants t 1 , t 2 . Between second and third instants t 2 , t 3 , reagent pressure pRea rises to just below predefined maximum pReaMax of reagent setpoint pressure pReaSW.  
         [0033]     Reagent pressure pRea has a pressure drop  62  between third and sixth instants t 3 , t 6 .  
         [0034]     The method according to the present invention functions as follows:  
         [0035]     The exhaust gas of internal combustion engine  10  contains undesired components such as nitrogen oxides. Exhaust gas treatment device  15  is responsible for reducing the undesired components as much as possible. Exhaust gas treatment device  15  is implemented as a catalytic converter, preferably as an SCR catalytic converter according to the related art cited at the beginning, for example. An SCR catalytic converter requires the reagent such as ammonia. Hydrocarbons may be provided as a reagent as a function of the design of the SCR catalytic converter. The ammonia may be obtained from a urea-water solution through hydrolysis. Possibly necessary hydrocarbons may be provided by fuel.  
         [0036]     In the exemplary embodiment shown, the reagent may be mixed in mixing chamber  37  with compressed air which is provided by compressed air path  40 . Compressed air path  40  and mixing chamber  37  may be dispensed with as a function of the concrete design. According to another embodiment, reagent introduction device  14 , which is simply a spray pipe, for example, may be identical to reagent dosing valve  36 . Reagent dosing valve  36  is then dispensed with. In this design, reagent dosing valve  36  is positioned directly in exhaust gas system  13 .  
         [0037]     Reagent pressure sensor  41  detects reagent pressure pRea in reagent path  31  between reagent pump  33  and reagent dosing valve  36 .  
         [0038]     Compressed air pump  50 , whose pressure is fixed at a predefined compressed air setpoint pressure pDISW using compressed air pump activation signal  51 , is provided in compressed air path  40 . The compressed air reaches mixing chamber  37  via compressed air valve  53 . Compressed air valve  53  is a check valve, for example, which has a flow-dependent opening pressure. According to an embodiment which is not shown in greater detail, compressed air valve  53  may be electrically actuated. Compressed air pressure sensor  52 , which detects compressed air pressure pDI in compressed air path  40 , is positioned between compressed air pump  50  and compressed air valve  53 .  
         [0039]     The following description is directed to a diagnosis of reagent path  31 . The diagnosis of compressed air path  40  is performed analogously. Reagent pump  33  contained in reagent path  31  corresponds to compressed air pump  50  positioned in compressed air path  40 . Reagent dosing valve  36  positioned in reagent path  31  corresponds to compressed air valve  53  positioned in compressed air path  40 , and reagent pressure sensor  41  positioned in reagent path  31  corresponds to compressed air pressure sensor  52  positioned in compressed air path  40 . Reagent setpoint pressure pReaSW predefined in reagent path  31  corresponds to compressed air setpoint value pDISW predefined in compressed air path  40 .  
         [0040]     A diagnosis through an evaluation of reagent pressure pRea provided by reagent pressure sensor  41  in regard to the absolute value and/or the changes first allows a check of whether there is a leak in reagent path  31 . A leak, in particular a leak which occurs in exhaust gas system  13 , may entail increased operating danger, in particular if the reagent is an easily flammable material such as fuel.  
         [0041]     Reagent pressure pRea is evaluated in comparator  42  by comparison to at least one predefined threshold value, which may be an absolute pressure and/or a pressure change and/or a pressure gradient. A targeted diagnosis, which extends to the proper functioning of reagent dosing valve  36  or another valve provided, may be performed via suitable fixing of diagnostic signal  49  by diagnostic controller  46 . Furthermore, reagent pump  33  may be checked for proper functioning.  
         [0042]     During normal operation of exhaust gas treatment device  15 , the dosing is performed using dosing signal  45 , which is supplied to reagent dosing controller  44 . The normal dosing operation may be interrupted by diagnostic signal  49 , which is provided by diagnostic controller  46 .  
         [0043]     It is first assumed that the normal dosing operation of the reagent is provided between first and second instants t 1 , t 2 . Setpoint default  43  outputs reagent pressure nominal value pReaN, which reagent pump activator  35  attempts to set via corresponding fixing of reagent pump activation signal  34 .  
         [0044]     A first diagnostic possibility is checking whether reagent pressure pRea at least approximately corresponds to predefined reagent nominal pressure pReaN. Comparator  42  compares reagent pressure pRea to a threshold value or multiple threshold values, which are related to predefined reagent nominal pressure pReaN. If a deviation is determined, error signal F is provided.  
         [0045]     Time curve  60  of reagent setpoint pressure pReaSW, which is preferably superimposed on predefined reagent nominal pressure pReaN, is predefined starting from second instant t 2 . A periodic, sinusoidal curve  60 , which may generally be provided without increased demands on reagent pump  33 , is provided in the exemplary embodiment shown. Reagent pump  33  must merely be capable of providing a reagent pressure pRea which exceeds reagent nominal pressure pReaN. Furthermore, a pulsed curve  60 , which places higher demands on reagent pump  33 , is suitable. Purely in principle, any curve  60  which has a chronological change may be provided.  
         [0046]     It is first assumed that reagent dosing valve  36  is at least partially open during normal dosing operation. Reagent pressure pRea is evaluated in that comparator  42  compares measured reagent pressure pRea to at least one threshold value, preferably to a time curve of the threshold value corresponding to curve  60 , the threshold value or the threshold value curve being adapted to predefined curve  60  and possibly to reagent nominal pressure pReaN. If the signal curve of reagent pressure pRea shown in  FIG. 2   b  occurs, in which predefined time curve  60  may no longer be found again at least from third instant t 3 , a reagent dosing valve  36  which sticks in the closed state must be assumed.  
         [0047]     Another diagnosis provides that reagent dosing valve  36  is closed at second instant t 2  or at the latest at third instant t 3 . If reagent path  31  is in working order, reagent pressure pRea rises to predefined maximum pReaMax of reagent setpoint pressure pReaSW, as shown in  FIG. 2   b , and subsequently remains there. For the diagnosis, it is sufficient if comparator  42  checks whether a pressure drop of reagent pressure pRea has occurred at at least one predefined instant, for example, at fourth instant t 4 , at which relative zero crossing  61  lies, and/or at fifth instant t 5 , at which minimum pReaMin lies. Comparator  42  may relate the threshold value to maximum pReaMax, for example. If no pressure drop or only a small permissible pressure drop is determined, reagent path  31  is in working order. If predefined time curve  60 , possibly having significantly reduced amplitudes, may be found again, reagent dosing valve  36  sticks in the at least partially open state. If there is a leak in reagent path  31 , a reagent pressure pRea will occur as shown as an example in  FIG. 2   c.    
         [0048]     The situation may first occur in which reagent pressure pRea does not rise to expected maximum pReaMax at third instant t 3 . If this is the case, a larger leak must be assumed, in which reagent pump  33  is no longer capable of maintaining predefined reagent setpoint pressure pReaSW.  
         [0049]     After third instant t 3 , i.e., after reaching maximum pReaMax, pressure drop  62  exists, which is ended in the exemplary embodiment shown from sixth instant t 6  of predefined time curve  60  by the pressure increase. In addition to a comparison with a threshold value, for example, at fourth and/or fifth instant t 4 , t 5 , a pressure gradient which may preferably be implemented as at least one differential quotient may be provided as a threshold value.  
         [0050]     If comparator  42  determines the threshold has been exceeded at least once, error signal F is provided and may be stored in a fault memory (not shown in greater detail) or displayed, for example.