Abstract:
A fill level detection apparatus ( 23 ) is introduced for ascertaining a fill level in a storage tank ( 19 ) for a liquid reducing agent for use in an exhaust gas aftertreatment device ( 3 ), having a fill level detector ( 21 ) and a driving and evaluating unit ( 22 ), which are connected to each other with electrical lines ( 47   a   , 47   b   , 47   c ) and with respect to DC-currents are decoupled from each other by capacitors ( 45 ), which are connected in series with said electrical lines, and with respect to AC-currents are coupled with each other. Said fill level detection apparatus is thereby characterized in that the capacitors ( 45 ) are disposed as part of the fill level detector ( 21 ) separated from the driving and evaluating unit ( 22 ) by the electrical lines ( 47   a   , 47   b   , 47   c ).

Description:
[0001]    This application claims benefit of Serial No. 10 2009 020 770.8, filed 6 May 2009 in Germany and which application is incorporated herein by reference. To the extent appropriate, a claim of priority is made to the above disclosed application. 
       BACKGROUND 
       [0002]    The invention relates to a fill level detection apparatus for ascertaining a fill level in a storage tank for a liquid reducing agent for use in an exhaust gas aftertreatment device. 
         [0003]    Fill level detection apparatuses for ascertaining a fill level in a storage tank for a liquid reducing agent to be introduced into an exhaust gas aftertreatment device are known from the market. Exhaust gas aftertreatment devices are required by law. They reduce the environmentally harmful nitrogen oxides contained in the untreated emissions of an internal combustion engine. In, for example, a method for selective catalytic reduction (SCR), liquid reducing agents, as, for example, a urea-water solution, are used. In the SCR method, ammonia is released from the urea-water solution by means of a hydrolysis reaction. Said ammonia reduces environmentally harmful nitrogen oxides to harmless nitrogen and water in a catalytic converter in the exhaust gas tract of the internal combustion engine. This so-called SCR method for purifying exhaust gas is known from the technical field. 
         [0004]    The fill level detection apparatus is used to acquire the fill level in the storage tank for the liquid reducing agent (for example so-called “Adblue”) and to elicit a timely refilling of the reducing agent. Fill level detectors are thus, for example, in use, which have measuring electrodes of various lengths that are submerged in the reducing agent. With the aid of electrical measurements, which utilize the electrical conductivity or the capacitive properties of the reducing agent, the fill level can be ascertained by virtue of the fact that detection is made of which electrodes are still submerged in the reducing agent. This is than an indication of the fill level. The measurements are preferably carried out with a pulse-width modulated (PWM) method with signal pulses that are as short as possible and with a small duty cycle in order to prevent a possible electrolysis of the reducing agent. The electrolysis would change the reducing agent (for example urea) and the electrodes, which would render the reducing agent unusable for its actual use in the catalytic converter and place wear on the electrodes. In addition, hydrogen would develop from the electrolysis, which in connection with oxygen forms explosive oxyhydrogen. 
         [0005]    The known fill level detection apparatuses preferably work with a capacitive sensor, which requires a capacitive decoupling in the control unit. These fill level detection apparatuses have the disadvantage that in the event of a fault, for example a short circuit to a battery voltage, they can only protect the fill level detector in a very complex manner. This fault can result from, for example, the insulation being rubbed off on the electrical feeder cables to the fill level detector or by other influences caused by a fault. The short circuited current can thereby initially flow unhindered into the fill level detector and can flow to ground via the electrically conductive fluid, which can lead to an undesirable electrolysis of the reducing agent. This is stopped after a diagnosis is made in the evaluating unit by an additional circuit and software function in said evaluating unit. With regard to the software function required for this operation, standard applications can not be relied upon, and a customized function has to be developed for this special case. This increases costs. The electronic evaluating unit is protected from a short circuit at the input in a known fashion by a capacitor, which is disposed at the input to the evaluating unit. 
       SUMMARY 
       [0006]    It is the aim of the invention to specify a reliably working, cost effective fill level detection apparatus, wherein an electrolysis of the reducing agent in the storage tank is prevented in each case by simple means. 
         [0007]    The aim is met according to the invention by virtue of the fact that the capacitor is disposed as part of the fill level detector separated from the driving and evaluating unit by the electrical wiring. The capacitor is thereby disposed at the input to the fill level detector and serves as a safeguard from direct current, for example in the case of a short circuit to the battery, which could occur in the event of a fault. The idea underlying the invention is on the one hand to protect the fill level detector from short circuit current (direct current) by a simple, discrete component, namely the capacitor, and on the other hand to protect the driving and evaluating unit by simple software measures because the short circuit current substantially corresponds to a predefinable case and is therefore easy to diagnose with a standard software. When a fault is detected, safeguards, which now only have to affect the driving and evaluating unit, can then be taken by software decisions in the driving and evaluating unit. As a first safeguard, a switch in the feeder line to ground can thus, for example, be opened. The use of the capacitor on the input side of the fill level detector is possible because the ascertainment of the fill level in the storage tank is conducted in a known manner with a pulse-width modulated signal (for example at approximately 5 kHz). The capacitor can thereby be dimensioned such that it is of low resistance at this frequency, i.e. is conducting. By the use of standard software for fault recognition and the capacitor for decoupling, the invention is simple to implement and is also for this reason cost effective. 
         [0008]    Important characteristics for the invention can furthermore be found in the following description and in the drawing. In so doing, the characteristics can be important for the invention individually as well as in various combinations without explicit reference being made in each case to this fact. Advantageous modifications are found in the sub-claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    An embodiment of the invention is exemplarily explained below with the aid of the figures. The following are shown: 
           [0010]      FIG. 1  is a schematic depiction of an internal combustion engine with an exhaust gas aftertreatment device; 
           [0011]      FIG. 2  is a detailed depiction of a fill level detector and a driving and evaluating unit from  FIG. 1 ; 
           [0012]      FIG. 3  is a schematic diagram of the fill level detector and the driving and evaluating unit from the technical field; 
           [0013]      FIG. 4  is a schematic diagram of the fill level detector according to the invention and driving and evaluating unit; and 
           [0014]      FIG. 5  is a diagram with different possible current levels in the fill level detector. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An internal combustion engine  1  having an exhaust gas aftertreatment device  3  is schematically depicted in  FIG. 1  in a greatly simplified manner. The exhaust gas aftertreatment device  3  has an exhaust pipe  5 , an oxidation catalytic converter  7  and a SCR catalytic converter  11 . A particle filter is not depicted, which normally is disposed downstream of the oxidation catalytic converter  7 . The direction of flow of the exhaust gas through the exhaust pipe  5  is indicated by arrows (without reference numerals). 
         [0016]    In order to supply the SCR catalytic converter  11  with a reducing agent, for example a urea-water solution, a spray pipe  13  for the urea-water solution is disposed in the exhaust pipe  5  upstream of the SCR catalytic converter  11 . If needed, the urea-water solution is injected via said spray pipe  13  by means of compressed air into the exhaust pipe  5  upstream of the SCR catalytic converter  11 . The spray pipe  13  is connected to the metering module  15  via a metering line  14 . In addition, a pressure line  16  for supplying compressed air from a compressed air generation unit  17  or a compressed air tank is provided at the metering module  15 . 
         [0017]    Beside the spray pipe  13 , the metering line  14  and the metering module  15 , the entire metering system comprises a metering pump  18  as well as a storage tank  19  for the urea-water solution. The storage tank  19  and the metering pump  18  as well as the metering pump  18  and the metering module  15  are in each case hydraulically connected to each other by lines (without reference numerals) so that the metering pump  18  supplies the metering module  15  with reducing agent from the storage tank  19 . 
         [0018]    A fill level detector  21  is disposed at the storage tank  19 . The fill level detector  21  is electrically connected to a driving and evaluating unit  22 . The driving and evaluating unit  22  can be a separate unit, it can however also be integrated in a control unit  29  of the internal combustion engine  1 . The fill level detector  21  and the driving and evaluating unit  22  together form the fill level detection apparatus  23 . 
         [0019]    For the sake of completeness, the sensors disposed in the exhaust gas system should be mentioned, namely a nitrogen oxide sensor  25  as well as temperature sensors  24  and  27 , with which the state of the exhaust gas is acquired. These sensors  24 ,  25  and  27  are connected to the control unit  29 , which in addition controls the internal combustion engine  1 , via signal lines (without reference numerals). 
         [0020]      FIG. 2  shows in detail the fill level detection apparatus  23  in a preferred embodiment. The fill level detector  21  essentially comprises four connecting lines  31  to four electrodes  33 ,  35   a ,  35   b ,  35   c , which are submerged in the storage tank  19 . The electrode depicted on the left side in  FIG. 2  thereby represents a base electrode  33 . The additional electrodes  35   a ,  35   b ,  35   c  are disposed parallel to the base electrode  33  and represent the measuring electrodes  35   a ,  35   b ,  35   c . The three measuring electrodes  35   a ,  35   b ,  35   c  have in each case a different length, a first measuring electrode  35   a  having the same length as the base electrode  33  and extending approximately to a base surface of the storage tank  19 . A second measuring electrode  35   b  is slightly shortened in comparison to the first measuring electrode  35   a . A third measuring electrode  35   c  extends only a slight way into the storage tank  19 . 
         [0021]    The four connecting lines  31  are fed to the fill level detector  21 . The connection of the base electrode  33  is then directly looped through the driving and evaluating unit  22  via a first cable connection  37  and then led further via a switch  39  to an electrical reference point  41  (for example ground). The connections of the measuring electrodes  35   a ,  35   b ,  35   c  have in each case a constant resistance  43  in the direction of the base electrode  33  and are directed in the fill level detector  21  to the driving and evaluating unit  22  in each case via a capacitor  45  and in each case a second cable connection  47   a ,  47   b ,  47   c . The driving and evaluating unit  22  substantially comprises a pulse-width modulator  49  for each measuring electrode  35   a ,  35   b ,  35   c , a signal shaping network  50  and an analog-digital converter  51 .  FIG. 2  shows for the sake of clarity only a pulse-width modulator  49 , a signal shaping network  50  and an analog-digital converter  51 , which are connected up to the line  47   b . A processor  53  activates the pulse-width modulator  49  and receives signals from the analog-digital converter  51 . The signal shaping network is connected up to the associated measuring electrode, in this instance the measuring electrode  35   b  so that the signal going out from the pulse-width modulator  49  is changed as a function of electrical influences, which are fed in via the measuring electrode  35   b . The electrical influence primarily reflects whether the measuring electrode  35   b  is submerged in the fluid and is thereby conductively connected to the base electrode  33  via said fluid. Using this information, a rough item of information about the fill level is indirectly reflected in the signal, which is present at the analog-digital converter  51  and is provided to the processor  53  for evaluation in digital form. 
         [0022]    The processor  53  is equipped for the purpose, particularly programmed for the purpose, of activating the measuring electrodes  35  and of evaluating the digital signal received from the analog-digital converters  51 . It is particularly programmed for the purpose of evaluating the signals for controlling the method of the fill level detection with a fault recognition routine. 
         [0023]      FIG. 3  schematically shows the fill level detection apparatus  23  from the technical field. It is thereby the case that such elements and regions, which are functionally equivalent to the elements of  FIG. 2 , bear the same reference numerals and are not once again explained in detail. The fill level detector  21  is together with the electrodes  33  and  35  symbolically depicted by a single fixed resistor  43  and a capacitor connected in parallel to it. In contrast to  FIG. 2 , the fill level detector  21  of  FIG. 3  does not have the capacitor  45 . This capacitor  45  is disposed in the technical field in the driving and evaluating unit  22  at the input of said driving and evaluating unit  22 . 
         [0024]    According to the invention, the capacitor  45  is in each case transferred from the driving and evaluating unit  22  into the fill level detector  21 .  FIG. 4  shows the situation according to the invention. 
         [0025]    The fill level detection apparatus  23  from the technical field works according to the following method: it is thereby assumed that the fill level detector  21  has the same electrode arrangement, as it is depicted in  FIG. 2 . Reference is therefore initially made to  FIG. 2  for the explanation of the method. The liquid, electrically conductive reducing agent constitutes an electrical resistor  55 , whose resistance is substantially smaller than the constant resistance  43 , between the base electrode  33  and one of the measuring electrodes  35  when the corresponding measuring electrode is submerged in said reducing agent. As seen electrically both resistors  43  and  55  are connected in parallel, which means that the total resistance of the two resistors  43  and  55  connected virtually in parallel is substantially smaller than the constant resistance  43 . Because measuring electrodes  35  of different lengths having respectively separate cable connections  47  are submerged in the reducing agent, the driving and evaluating unit  22  can ascertain a fill level, particularly a critical fill level, because only the constant resistance  43  takes effect for the driving and evaluating unit  22  when the measuring electrodes  35  are not submerged in the reducing agent and the total resistance from the resistors  43  and  55  connected in parallel takes effect when the measuring electrodes  35  are submerged in said reducing agent. 
         [0026]    It is necessary for the measurement of the resistance that a voltage is applied between the base electrode  33  and the measuring electrodes  35 , whereby a current flows through the reducing agent. In order to prevent an electrolysis of the reducing agent by the measuring current, the resistance measurement is carried out with a pulse-width modulated signal having a small duty cycle so that the signal pulse is very short. 
         [0027]    As a result of a fault in the region of the fill level detection apparatus  23 , for example resulting from the insulation of the electrical feeder cable  47  to the measuring electrodes  35  being rubbed off, it is possible for a short circuit to occur and for a battery current (direct current) to be supplied to the electrical feeder cable  47  and in so doing also to the fill level detector  21 . This would result in a lasting electrolysis of the liquid reducing agent. The driving and evaluating unit  22  is protected by the arrangement of the capacitor  45  according to the invention because said driving and evaluating unit  22  DC decouples the measuring electrodes  35   a ,  35   b ,  35   c  from the lines  47 . 
         [0028]    In other words, the arrangement of the capacitor  45  at the input of the fill level detector  21  prevents an electrolysis of the reducing agent in the storage tank  19  when a fault occurs. In the case of a fault, the short circuit current now present at the driving and evaluating unit  22  constitutes a substantially predefinable condition, for example at the input of the analog-digital converter, which can be recognized in the driving and evaluating unit  22  by a specific query in the software. The protective mechanisms are then initiated by software controlled actions, like, for example, an opening of the switch  39 . 
         [0029]      FIG. 5  shows a diagram with different current levels, which can occur in the driving and evaluating unit  22  during the operation and which are diagnosed by the method according to the invention. The current I 1  constitutes the current, which flows between a measuring electrode not submerged in the reducing agent via a fixed resistor  43 . Current I 2  constitutes the current, which flows between a measuring electrode submerged in the reducing agent and the parallel connected electrical resistor  55  of the reductant fluid via the fixed resistor  43 . It should be noted at this point that the depiction of  FIG. 5  is purely qualitative and that I 1  and I 2  would depict a pulse-width modulation upon a pulse-width modulated signal being generated.  FIG. 5  however illustrates well the qualitative effect of the resistance which varies with the fluid level. 
         [0030]    As a result of the separate electric power supply of the individual measuring electrodes to the driving and evaluating unit  22 , it is possible to identify the measuring electrodes  35 , which are still submerged in the reducing agent and—if necessary in the case of a low fill level of the reducing agent having been diagnosed—to emit an optical or acoustic warning signal, for example on the dashboard of the motor vehicle if only two or even one single measuring electrode(s)  35  still are submerged in the reducing agent. I max  shows the short circuit current which occurs in the case of a fault, from which the fill level detector  21  is on the one hand protected by the capacitor  45 , and on the other hand the driving and evaluating unit  22  recognizes the fault and initiates appropriate safeguards as previously described.