Abstract:
A method for operating an exhaust gas system of an internal combustion engine is described, wherein NOx is reduced by means of a SCR catalyst and a NOx reduction capability of an aqueous urea solution to be introduced into the exhaust gas system is monitored, and wherein at least one first variable characterizing the ammonia content of the water is ascertained and an ageing of the aqueous urea solution is inferred from said first variable.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for operating an exhaust gas system of an internal combustion engine. 
         [0002]    Internal combustion engines, particularly diesel engines which reduce nitrogen oxides (NOx) present in the exhaust gas of the internal combustion engine by means of selective catalytic reduction (SCR), are known from the market. The NOx reduction takes place, for example, by means of ammonia (NH3) being introduced into the exhaust gas or respectively into the SCR catalyst. A patent publication from this subject area is, for example, DE 10 2009 029 107 A1. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention has, among others, the advantage that an ageing of an aqueous urea solution for reducing nitrogen oxides in the exhaust gas of an internal combustion engine can be determined. A NOx reduction capability of the aqueous urea solution can especially be determined from a first variable characterizing the ammonia content in the water and a second variable characterizing the composition of said aqueous urea solution. Said second variable relates to a, if not the, essential variable characterizing the ageing. The aqueous urea solution can be optimally metered by means of the determined NOx reduction capability, and nitrogen oxides in the exhaust gas can be chemically broken down substantially independently of the quality and/or ageing of said aqueous solution. Components of a metering device and/or a feed device of the aqueous urea solution can furthermore be preventively protected against a chemical degradation. 
         [0004]    The invention, in certain embodiments, relates to a method for operating an exhaust gas system of an internal combustion engine, wherein NOx (nitrogen oxides) are reduced by means of an SCR catalyst (“selective catalytic reduction”) and a NOx reduction capability of an aqueous urea solution to be introduced into the exhaust gas system is monitored (and therefore ascertained). The NOx reduction capability describes a reactivity of the aqueous urea solution to the reduction of NOx by means of the SCR catalyst. After being injected into the exhaust gas system, said aqueous urea solution releases ammonia (NH3) which is used in a known manner to chemically reduce nitrogen oxides. Said aqueous urea solution can deviate from specified properties for different reasons, whereby the reduction of nitrogen oxides in the SCR catalyst can be impaired. As the case may be, this can lead to the internal combustion engine—and therefore an associated motor vehicle—being shut down by an automatic switch-off after a certain time interval has elapsed. Besides an intentional manipulation on the part of the driver as, for example, an intentional dilution of the urea solution or an unintentional improper filling of a storage reservoir, the ageing of the aqueous urea solution can also be the reason for a decrease in the NOx reduction capability of the system. The ageing is characterized by a change in the composition of the aqueous urea solution and can happen relatively quickly, in particular in the case of high temperatures. 
         [0005]    According to the invention, at least one first variable characterizing the ammonia content in the water is ascertained. The ageing of the aqueous urea solution (“solution”) can be inferred from the first variable which was ascertained. The ascertained variable is then used when monitoring the NOx reduction capability. The first variable characterizing the ammonia content in the water provides specific information with regard to the ageing of the solution. When the aqueous urea solution ages, the urea breaks down, wherein ammonia is formed, which can be easily dissolved in the surrounding aqueous solution. Ammonia dissolved in an aqueous solution has a reduction capability for the NOx reduction which is comparable to the ammonia bound in the urea. A certain amount of water as well as a certain amount of ammonia evaporates as a function of the properties of the storage reservoir in which the aqueous urea solution is stored. When said aqueous urea solution ages, the NOx reduction capability is then changed substantially as a function of the evaporation of the water and that of the released ammonia. The respective evaporation rates can vary and as the case may be are difficult to estimate or ascertain. Thanks to the method according to the invention, this no longer presents a problem. 
         [0006]    The inventive method furthermore makes it possible for a metering system for metering and injecting the solution into the exhaust gas to be preventively protected. Ammonia reacts in a highly alkaline manner in the aqueous solution. With increasing ammonia content (NH3 concentration), the urea solution acts in a correspondingly aggressive manner on the components of the metering system. An ammonia content of 2 percent can, for example, represent a critical threshold value for possible damage to components. By determining the first variable characterizing the ammonia content, the driver of the motor vehicle can be warned and counter measures can be put into effect. In so doing, damage is avoided and costs are reduced. 
         [0007]    Provision is further made in the method according to the invention for at least one second value characterizing the composition of the aqueous urea solution to be ascertained and for the NOx reduction capability of said aqueous urea solution to be inferred from the first and the second variable. The “composition” essentially means the concentration of urea in the solution, i.e. the proportion of urea in the total quantity, as well as an unknown proportion of ammonia which is dissolved in the water. The composition ascertained in this way is however non-specific with respect to the NOx reduction capability of the entire solution because the proportion of the ammonia dissolved in the water due to ageing cannot be detected or not correctly detected. By solely determining the second variable, a sufficiently precise result can thus only be delivered for solutions which have not aged. Without the additional information provided by the first variable, a supposedly insufficient quality for the purpose of NOx reduction could be detected in the case of a severe ageing of the aqueous urea solution. For that reason, the first variable, which characterizes the ammonia content in the water, and the second variable, which characterizes the composition of the aqueous urea solution, are thus ascertained. With both variables together, the NOx reduction capability of the aqueous urea solution, i.e. the quantity of ammonia effective for reducing NOx, can be inferred with a high degree of accuracy. 
         [0008]    Provision is made in one embodiment of the invention for the first variable to be an electrical conductivity of the aqueous urea solution. As a result of the dissociation in the water, an equilibrium develops between the ammonia (NH3) and NH4+ ions formed in the water. Said ions have a comparatively high level of mobility and contribute accordingly to the conductivity of the solution. The proportion of the NH4+ ions is inferred from the conductivity which has been determined, and the proportion of ammonia (NH3) is inferred from said proportion of the NH4+ ions. The ageing of the aqueous urea solution is in turn inferred from said proportion of ammonia (NH3). The electrical conductivity therefore characterizes said ageing of the aqueous urea solution particularly well. 
         [0009]    Provision is made in a further embodiment of the invention for the second variable to be a density and/or a refractive index and/or a sound velocity and/or a thermal conductivity and/or a dielectric permittivity of the aqueous urea solution. Previously known methods and elements can be used to determine the aforementioned values for the second variable, whereby costs can be saved. 
         [0010]    Provision is furthermore made by the method for the ascertained first and second variable to be linked to one another by means of at least one characteristic diagram and/or a table and/or a mathematical formula in order to infer the NOx reduction capability of the aqueous urea solution. The NOx reduction capability can generally be expressed as: 
         [0000]        R=f ( c _NH3_urea+ c _NH3),  (1)
       wherein   c_NH3_urea—NH3 concentration bound in urea;   c_NH3=NH3 concentration dissolved in an aqueous solution       
 
         [0014]    The NOx reduction capability “R_new” without the influence of ageing can be specified in the following way: 
         [0000]        R _new= f ( c _NH3_urea_new),  (2)
       wherein   c_NH3_new=NH3 concentration bound in urea without ageing.       
 
         [0017]    The ageing process can be specified in the following way: 
         [0000]        c _NH3_urea= c _NH3_urea_new− f 1(ageing),  (3)
       wherein   f 1 =function, which describes the change in the NH3 concentration bound in the urea due to ageing.       
 
         [0000]        c _NH3 =f 2(ageing),  (4)
       wherein   f 2 =function, which describes the NH3 concentration dissolved in an aqueous solution as a result of ageing.       
 
         [0022]    On account of the unknown evaporation, which is a function of the storage reservoir (“tank system”), the functions f 1  and f 2  or respectively the amounts thereof are generally not the same. For that reason, it can be stated: 
         [0000]      f1□−f2,  (5)
       wherein the symbol “□” means “does not equal”.       
 
         [0024]    A primary measurement of quality can be specified as follows: 
         [0000]        Q _mess_prim= f ( c _urea, c _NH3),  (6)
       wherein   Q_mess_prim corresponds to the density or the refractive index of the solution and thereby to the second variable;   c_urea=urea concentration of the solution.       
 
         [0028]    The equation (6) is, for example, ascertained by means of the density or the refractive index of the aqueous urea solution and cannot selectively describe the NOx reduction capability of said aqueous urea solution. Additional options for a “concentration measurement”, which are mentioned here by way of example, include the sound velocity, the thermal conductivity or the dielectric permittivity. For example, an ageing of the aqueous urea solution falsifies the variable Q_mess_prim. The equation (6) alone is therefore only sufficiently exact for a solution that has not aged having c_NH3=0. In order to determine the amount of ageing, an ageing measured variable A_mess can be specified as follows: 
         [0000]        A _mess= f ( c _NH3),  (7)
       wherein   A_mess corresponds to the electrical conductivity of the solution and therefore to the first variable.       
 
         [0031]    The ageing of the aqueous urea solution can be selectively ascertained using the equation (7). The NOx reduction capability which results in total can be specified as follows by means of the equations (6) and (7): 
         [0000]        R=f ( Q _mess_prim, A _mess)  (8)
 
         [0032]    The NOx reduction capability of the aqueous urea solution can thereby be expressed as a two dimensional function of the primary measured variable Q_mess_prim according to equation (6) and the ageing measured variable A_mess according to equation 7. 
         [0033]    The relationships formulated by means of the equations can preferably be described in an open-loop and/or closed-loop control device of an internal combustion engine or exhaust gas system by means of one or a plurality of characteristic diagrams, tables and/or mathematical formulas. The first and the second variable and consequentially the state or respectively the NOx reduction capability of the aqueous urea solution can thereby be particularly easily and exactly ascertained. 
         [0034]    Provision is made in one embodiment of the method for a filling level of a reservoir of the reducing agent and/or a point in time of a filling of the reservoir and/or a temperature of the reducing agent to be used in order to ascertain the NOx reduction capability. In so doing, the accuracy in ascertaining the first and second variable can if necessary be improved or the result can be subjected to a plausibility check. 
         [0035]    The invention is particularly useful if a quantity of the aqueous urea solution to be introduced into the exhaust gas system is ascertained as a function of the determined NOx reduction capability of said aqueous urea solution. Hence, the reduction of the nitrogen oxides resulting in the SCR catalyst can also then take place with a quantity of ammonia which is optimal in each case if the aqueous urea solution has already aged. In so doing, a pilot control for the metering of the aqueous urea solution can be manipulated such that a respectively changed NO reduction capability is compensated by a correspondingly adapted metered quantity. This facilitates using the aqueous urea solution over a comparatively large reactivity range without having to change the solution. Effort and expense can thereby be saved and the operation of the exhaust gas system will become more robust. 
         [0036]    The method according to the invention can be particularly simply and cost effectively carried out by means of a computer program which, for example, can be executed in an open-loop and/or closed-loop control device for an internal combustion engine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Exemplary embodiments of the invention are subsequently explained with reference to the drawings. In the drawings: 
           [0038]      FIG. 1  shows a simplified illustration of an internal combustion engine and an exhaust gas system; and 
           [0039]      FIG. 2  shows a flow diagram for ascertaining a NOx reduction capability of an aqueous urea solution. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    The same reference numerals are used in all of the figures for functionally equivalent elements and variables, even when the embodiments are different. 
         [0041]      FIG. 1  shows a simplified diagram of an exhaust gas system  10  of a motor vehicle in the lower section of the drawing. An internal combustion engine  12  is symbolically depicted on the left above the exhaust gas system, said combustion engine discharging exhaust gas into the exhaust gas system  10  via a pipe connection  14 . An open-loop and/or closed loop control device  16  is connected via incoming and outgoing electrical lines  20  and  22  to said combustion engine  12  as well as via incoming and outgoing electrical lines  24  and  26  to components of the exhaust gas system  10 . The connections are merely indicated in the drawing. The open-loop and/or closed-loop control device  16  further comprises a computer program  18  and one or a plurality of characteristic diagrams  21 . The computer program  18  can exchange data with the characteristic diagram  21 . 
         [0042]    The exhaust gas is substantially passed through the exhaust gas system  10  and processed therein from left to right. The present embodiment relates to the exhaust gas system of a diesel motor vehicle. In the direction of flow of the exhaust gas, said exhaust gas system  10  comprises therefore a diesel oxidation catalyst  28 , a diesel particle filter  30 , a feed device  31  for an aqueous urea solution  33  and a SCR catalyst (SCR meaning “selective catalytic reduction”). 
         [0043]    A lambda probe  34  is disposed in the exhaust gas flow upstream of the diesel oxidation catalyst  28 . A NOx sensor  36  is disposed in the exhaust gas flow in each case upstream and downstream of the SCR catalyst  32 . In addition, the exhaust gas system  10  comprises four temperature sensors  38  in the present embodiment. The temperature sensors  38 , the lambda probe  34  and the NOx sensors  36  are connected to the open-loop and/or closed loop control device  16  via the incoming and outgoing electrical lines  24  and  26 . This is however not shown in detail in the drawing of  FIG. 1 . 
         [0044]    A storage reservoir  40 , which contains the aqueous urea solution and is connected to the feed device  31  via a hydraulic line  41 , is disposed in the upper right region of the drawing of  FIG. 1 . A measuring device  42 , which can ascertain physical variables of the aqueous urea solution  33 , is depicted to the left of the storage reservoir  40 . The measuring device  42  is electrically connected to the open-loop and or closed-loop control device  16  via electrical lines  44  and  46 . A temperature sensor  47  ascertains the temperature of the aqueous urea solution  33  in the storage reservoir  40 . 
         [0045]    During the operation of the exhaust gas system  10 , the aqueous urea solution  33  is injected in metered doses into said exhaust gas system  10  by means of the feed device  31 . A reduction of the nitrogen oxides contained in the exhaust gas, which takes place in the SCR catalyst  32 , is controlled and monitored by means of the NOx sensors  36  as well as by means of the temperature sensors  36 . The measuring device  42  continuously or occasionally ascertains an electrical conductivity  48  and a density of the aqueous urea solution  33 . Said measuring device  42  can alternatively ascertain a refractive index of said aqueous urea solution  33 . 
         [0046]    The electrical conductivity  48  obtained and the density  50  or respectively the refractive index obtained are transmitted to the open-loop and/or closed-loop  16  control device  16 . The computer program  18  determines a first variable  52 , which characterizes the ammonia content (NH3 content) in the water of the aqueous urea solution  33 , from the electrical conductivity  48 . In addition, the computer program  18  determines a second variable  54 , which characterizes the composition of said aqueous urea solution  33 , from the density  50 . In a complementary manner, the determination of the first and second variable  52  and  54  can take place while taking into account the temperature of said aqueous solution  33  provided by the temperature sensor  47 . 
         [0047]    A NOx reduction capability is subsequently inferred from the first variable  52  and the second variable  54 . For that purpose, the characteristic diagram  21  comprises on the one hand functional relationships between the first and second variable  52  and  54  and on the other hand the functional relationship for the NOx reduction capability. Said functional relationship for the NOx reduction capability is generally expressed as a function of the respective concentrations: 
         [0000]        R=f[f ( c _urea, c _NH3),  f ( c _NH3)],       wherein   c_urea=concentration of urea   c_NH3=NH3 concentration dissolved in an aqueous solution         
         [0051]      FIG. 2  shows a flow diagram for operating the exhaust gas system  10  of the internal combustion engine  12 . The NOx reduction capability and the corrected metered quantities of the aqueous urea solution  33  ensuing therefrom are especially ascertained. The procedure depicted in  FIG. 2  begins in a starting block  58 . 
         [0052]    The electrical conductivity  48  of the aqueous urea solution  33  is ascertained in a first block  60 . In a subsequent block  62  the first variable  52 , which characterizes the ammonia content of the water, is ascertained from the electrical conductivity  48 . 
         [0053]    In a further block  64 , the density  50  of the aqueous urea solution  33  is ascertained. In a subsequent block  66 , the second value, which characterizes the composition of the aqueous solution  33 , is ascertained from the density  50 . 
         [0054]    In a further block  68 , the temperature of the aqueous urea solution  33  is ascertained. In a complementary manner, a filling level and/or a point in time of the last filling of the storage reservoir  40  can be ascertained and used to determine the plausibility of the succeeding operations. The temperature and the first as well as the second variable  52  and  54  are linked to one another by means of the characteristic diagram  21  and through the use of mathematical formulas, and a measurement for the NOx reduction capability of the aqueous urea solution  33  is ascertained therefrom. If said NOx reduction capability is less than a predeterminable threshold value, an error bit can be placed in a supplementary manner in a diagnostic memory and/or an item of information can appear on the dashboard of the motor vehicle. 
         [0055]    In a further block  70 , a metered quantity of the aqueous urea solution  33  is ascertained as a function of the previously obtained measurement of the NOx reduction capability. Said metered quantity can be used during the operation of the exhaust gas system  10  to inject a corresponding quantity of said aqueous urea solution  33  via the feed device  31  into said exhaust gas system  10 . The metering of the aqueous urea solution  33  can thereby be optimized in accordance with the NOx reduction capability ascertained; and therefore neither too small of nor too large of a quantity of ammonia is supplied to the SCR catalyst  32 . In a succeeding end block  72 , the procedure depicted in  FIG. 2  ends.