DE102015223689A1 - Method for calculating and applying a monitoring criterion - Google Patents

Abstract

The invention relates to a method for calculating a monitoring criterion, which is an indicator of the presence of a zeolite-containing SCR catalyst (4) in an exhaust line (1) of a motor vehicle, wherein the monitoring criterion is calculated by temporal temperature profiles upstream and downstream of the SCR catalyst (4) are compared. Furthermore, the invention enters a method for the application of the monitoring criterion, the method comprising the following steps: First, it is checked whether a heating condition for the SCR catalytic converter (4) is fulfilled. Subsequently, the calculation of the monitoring criterion and a quantity of water stored in the SCR catalytic converter (4) is started. In the next step, it is checked whether a sufficient amount of water has reached the SCR catalytic converter (4) since the engine started. At the end of the process, the presence of an SCR catalyst (4) in the exhaust line (1) is concluded.

Description

The present invention relates to a method for calculating and applying a monitoring criterion, which is an indicator of the presence of a zeolite-containing SCR catalyst in an exhaust system of a motor vehicle. Furthermore, the invention relates to two computer programs that perform each step of the inventive method for calculating and applying the monitoring criterion when they run on a computing device, as well as a machine-readable storage medium, which stores the computer programs. Finally, the invention relates to an electronic control unit which is set up to carry out the methods according to the invention.

State of the art

As more and more stringent NOx emission (NOx) limits are introduced, a variety of exhaust aftertreatment technologies are being developed to control NOx emissions in the exhaust of a diesel engine. One of these exhaust aftertreatment technologies is selective catalytic reduction in an SCR catalyst. The nitrogen oxides contained in the exhaust gas are reduced on the SCR catalytic converter by means of ammonia to nitrogen (N ₂ ). Although in the case of using an SCR catalyst with a remote engine design in the exhaust system, the aging of such an SCR catalyst is relatively limited, since the SCR catalyst is then exposed to high temperatures due to the hot hot exhaust gas, compliance with emission requirements is a monitoring of the functionality of the SCR catalyst required.

Today's SCR catalysts consist of several components. On a honeycomb-shaped carrier material is the so-called washcoat, which consists inter alia of zeolite. Zeolite is a crystalline, very finely porous material and is used for surface enlargement. In the washcoat or on its surface, the catalytically active metals, such as copper or iron, are embedded. It is known that zeolites can absorb water and other low molecular weight substances and release them again on heating, without their crystal structure being destroyed. A characteristic parameter for evaluating the effect of the adsorption and desorption effects of water on the zeolite is the adsorption enthalpy. It describes the increase or decrease in the energy content of the thermodynamic system exhaust gas zeolite / SCR catalyst.

In the DE 10 2009 007 763 A1 For example, a method for determining an efficiency of an SCR catalyst which can be charged with a reducing agent and which is arranged in an exhaust tract of an internal combustion engine is already disclosed. In this case, with the aid of the SCR catalyst, at least one substance of an exhaust gas mixture is sorbed and a loading state of the SCR catalyst with a substance of the exhaust gas mixture which is different from the reducing agent and which is sorbed by the SCR catalyst is determined. The method described is such that initially at least one parameter of the exhaust gas mixture is measured, which changes as a function of the effectiveness of the SCR catalytic converter during the flow through the exhaust gas mixture through the SCR catalytic converter. From the measured value thus acquired and a comparison value, a difference is formed and compared with a setpoint value.

Disclosure of the invention

In the inventive method for calculating a monitoring criterion, which is an indicator of the presence of a zeolite-containing SCR catalyst in an exhaust system of a motor vehicle, this monitoring criterion is calculated by comparing temporal temperature profiles upstream and downstream of the SCR catalytic converter. The use of the monitoring criterion according to the invention is particularly advantageous because the difficult-to-describe temperature effects due to adsorption and desorption of water on the zeolite need not be mapped in a model value, thus no temperature model is necessary for the presence control of the SCR catalyst. Furthermore, to determine the monitoring criterion, only one temperature sensor downstream of the SCR catalytic converter is necessary, so that the installation of a NOx sensor downstream of the SCR catalytic converter can be dispensed with. The latter would be more expensive and more expensive than the use of a temperature sensor.

In particular, the value of the monitoring criterion is calculated as a function of the amount of water that has reached the SCR catalyst. This procedure is very advantageous, since in this way the exo- and endothermic properties of the SCR catalyst are optimally taken into account in the adsorption and desorption of water, which have a significant influence on the temperature profiles. Will the timing of the comparison of the temperature curves upstream and downstream of the SCR catalyst too early The temperature differences may not be good enough, as the exothermicity of the water adsorption has not yet started properly. On the other hand, if one chooses the time of the comparison too late, a larger amount of water has already passed through the SCR catalyst and the adsorption and desorption effects may have already expired, resulting in only minor differences in the temperatures.

Advantageously, the value of the monitoring criterion is calculated as a function of the temperature of the internal combustion engine at startup. This particularly advantageous procedure ensures that the influence of the starting temperature of the internal combustion engine on the manifestation of the temperature effect is taken into account. If the internal combustion engine is restarted in the "warm" state, the temperature effect of the adsorption and desorption of water can be shorter and weaker, since at higher temperatures the two effects are superimposed or less pronounced. A clear indication of the presence of an SCR catalyst is therefore favored if the assessment of the monitoring criterion is carried out at low engine temperatures, preferably at a cold start.

According to one embodiment of the invention, the monitoring criterion is calculated by means of a cross-correlation of the temporal temperature profiles or the temporal temperature gradient profiles upstream and downstream of the SCR catalytic converter. In this case, the temperature gradient is understood to mean the bandpass-filtered temperature signal which corresponds to the low-pass filtered first derivative of the temperature signal. The calculation of the cross-correlation of the temporal temperature profiles is carried out as follows: CrssCorr (τ) = ∫ tEnd / 0y _SCRUs (t) · y _SCRDs (t - τ) dt (1) where CrssCorr is the cross-correlation, y _SCRUs the temperature _signal upstream of the SCR catalyst, y _SCRDs the temperature _signal downstream of the SCR catalyst, t _end the end time of the correlation _calculation , and τ the time offset between the two temperature _signals . The calculation of the cross-correlation of the temporal temperature gradient _{curves is} carried out analogously to formula (1), wherein only the temperature _signal upstream of the SCR catalyst y _SCRUs and the temperature _signal downstream of the SCR catalyst y _{SCRDs are} replaced by the respective temperature gradient signals. By means of the cross-correlation, a relationship between two or more temporal functions can be described advantageously. Thus, the result of the cross-correlation, which corresponds to the monitoring criterion, is a measure of how similar the two temperature curves are in dependence on a given time shift τ. Both in the calculation of the cross-correlation according to formula (1) and in the calculation of the other correlation functions given below, the amount of the result is evaluated in each case. The result is thus a value between 0 and 1 for the correlation functions.

wobei E_CrssCorr die Energiekreuzkorrelation ist und die anderen Variablen die gleiche Bedeutung haben wie in Formel (1). Für die Berechnung der Energiekreuzkorrelation der Temperaturgradientensignale gelten die gleichen Vorschriften, welche oben schon im Zusammenhang mit der Kreuzkorrelation genannt wurden.In another embodiment of the invention, the monitoring criterion is preferably calculated by means of an energy cross-correlation of the temporal temperature profiles or the temporal temperature gradient profiles upstream and downstream of the SCR catalytic converter. The normalized energy cross-correlation of the temperature signals upstream and downstream of the SCR catalyst is calculated as follows:

where E _{CrssCorr is} the energy _{cross-correlation} and the other variables have the same meaning as in formula (1). For the calculation of the energy cross-correlation of the temperature gradient signals, the same rules apply, which have already been mentioned above in connection with the cross-correlation.

wobei E² _CrssCorr die quadratische Energiekreuzkorrelation ist und die übrigen Variablen die gleiche Bedeutung haben wie in Formel (1) und (2). Auch im Fall der quadratischen Energiekreuzkorrelation kann durch einfaches Ersetzen die quadratische Energiekreuzkorrelation der Temperaturgradientensignale berechnet werden (vergleiche Vorschrift zur Berechnung der Kreuzkorrelation). Die quadratische Energiekreuzkorrelation wird vorteilhafter Weise zur vereinfachten Berechnung verwendet. Ist das Ergebnis der Kreuzkorrelation bzw. der Energiekreuzkorrelation gleich Null, so stehen die beiden miteinander verglichenen Temperatursignale in keiner Beziehung zueinander, d.h. sie sind nicht korreliert. Je größer der betragsmäßige Wert des Ergebnisses der Korrelationen ist, desto ähnlicher sind sich die beiden Temperatursignale. Erreicht das Ergebnis der Korrelationen den maximalen Wert Eins, so korrelieren die verglichenen Temperatursignale bei einer Verschiebung um die Totzeit τ. In yet another embodiment of the invention, the monitoring criterion is advantageously calculated by means of a quadratic energy cross-correlation of the temporal temperature profiles upstream and downstream of the SCR catalyst as follows:

where E ² _{CrssCorr is} the quadratic energy _{cross-correlation} and the remaining variables have the same meaning as in _formulas (1) and (2). Also in the case of the quadratic energy cross correlation, the quadratic energy cross correlation of the temperature gradient signals can be calculated by simple substitution (compare rule for calculating the cross correlation). The quadratic energy cross correlation is advantageously used for simplified calculation. If the result of the cross-correlation or the energy cross-correlation is equal to zero, then the two temperature signals compared with each other are not related to one another, ie they are not correlated. The larger the magnitude value of the result of the correlations, the more similar the two temperature signals. If the result of the correlations reaches the maximum value one, then the compared temperature signals correlate with a shift by the dead time τ.

Due to the characteristic temperature effects of the zeolite coating in the SCR catalyst, the temperature signals upstream and downstream of the SCR catalyst differ greatly in the adsorption and desorption phases. Therefore, a low result of the correlation, that is, a low value of the monitoring criterion, corresponds to the case that an SCR catalyst is present in the exhaust line. If no SCR catalyst is present in the exhaust line, or if its zeolite coating is no longer functioning properly, then the temperature signals are similar upstream and downstream of the SCR catalyst. This corresponds to a high correlation value.

Preferably, both the temperature profile upstream of the SCR catalyst and the temperature profile downstream of the SCR catalyst are measured with a temperature sensor. An advantage of this procedure is that in this way the difficult-to-describe temperature effects due to adsorption and desorption of water on the zeolite need not be modeled.

According to a further embodiment of the invention, the temperature profile upstream of the SCR catalyst is indicated by a calculated model temperature profile. This can advantageously be dispensed with the temperature sensor upstream of the SCR catalyst.

The inventive method for calculating the monitoring criterion runs in particular in several steps. First, it is checked whether a heating condition for the SCR catalyst is met. It is therefore checked at the beginning of each driving cycle, whether the SCR catalyst was operated in the previous driving cycle at sufficiently high temperatures and thus the bound in the zeolite coating water could be sufficiently expelled again. For this purpose, for example, the engine stop time, the start and / or end temperature of the system and / or the average or maximum temperature are used. The cumulative amount of heat that has passed through the SCR catalytic converter in the previous drive cycle can also be used as a measure of the heat-up state of the SCR catalytic converter. Subsequently, the calculation of the monitoring criterion and a quantity of water stored in the SCR catalytic converter is started. It is then checked whether a sufficient amount of water has reached the SCR catalytic converter since the engine started.

The method according to the invention for the application of the calculated monitoring criterion comprises concluding the presence of an SCR catalytic converter in the exhaust gas line after the steps of the method for calculating the monitoring criterion have been carried out. Using these two methods, it is advantageously possible to easily calculate a monitoring criterion by means of which the presence or functionality of an SCR catalytic converter in an exhaust system of a motor vehicle can be inferred, the temperature effects on the zeolite coating of the SCR catalytic converter be taken into account.

In particular, the presence of an SCR catalytic converter in the exhaust gas line is concluded when the monitoring _criterion has a lower value than a threshold value v _th . By means of the comparison of the monitoring criterion with the predefinable threshold value v _th , it is advantageously possible with little computational effort to make a statement as to whether or not an SCR catalytic converter is installed in an exhaust gas system.

In one embodiment of the method according to the invention for the application of the monitoring criterion, the absolute minimum of the monitoring criterion is determined at the end of the drive cycle. In this way, the computational effort is advantageously reduced because in a drive cycle, the value of the monitoring criterion no longer has to be compared multiple times with a threshold, but only once at the end of the drive cycle, the absolute minimum of the monitoring criterion is determined. This absolute minimum is then compared to a threshold again.

The invention furthermore comprises two computer programs which are set up to carry out each step of the first and / or the second method according to the invention, in particular if they are executed on a computing device or electronic control device. This allows the implementation of the method according to the invention on an electronic control unit, without having to make any structural changes thereto.

The invention also comprises a machine-readable storage medium on which the computer programs are stored, and an electronic control unit which is set up to carry out the methods according to the invention.

Further advantages and features of the invention will become apparent from the following description of embodiments in conjunction with the drawings. In this case, the individual features can be implemented individually or in combination with each other.

Brief description of the drawings

In the drawings show:

1 schematically an exhaust line and a control unit,

2 schematically the flow of the two methods according to a first embodiment of the invention,

3 schematically the flow of the two methods according to a second embodiment of the invention and

4 in two diagrams, a calculation of the correlation function by means of the quadratic energy cross-correlation function of the temperature gradient according to an embodiment of the invention.

embodiments

1 schematically shows an exhaust system 1 a motor vehicle (not shown) with a reducing agent metering 2 , an SCR on particulate filter (SCROF) 3 , an SCR catalyst 4 with a zeolite coating, a temperature sensor 5 upstream of the SCR catalyst 4 , a temperature sensor 6 downstream of the SCR catalyst 4 and a controller 7 , The control unit 7 has a data connection to the temperature sensors 5 . 6 on.

In 2 schematically a flow of the method for calculating the monitoring criterion according to a first embodiment of the invention is shown. In the first step 10 of the method, the motor vehicle is put into operation. In the following step 11 it is checked whether the heating condition for the SCR catalyst 4 is satisfied. It is therefore checked at the beginning of each driving cycle, whether the SCR catalyst 4 was operated in the previous cycle under sufficiently high temperatures and thus the bound in the zeolite coating water could be sufficiently expelled again. For this example, the engine shutdown time, the start temperature of the system and / or the average or maximum temperature are used. Also the accumulated amount of heat, the SCR catalyst 4 in the previous driving cycle, can be used as a measure of the heat-up state of the SCR catalytic converter 4 be used. If the heating condition is not met, then in step 12 the message is issued that in this driving cycle no diagnosis of the SCR catalyst 4 can be made.

However, if the baking condition is met, a sufficiently pronounced temperature effect due to water adsorption and desorption is to be expected. In this case, in step 13 the temperature signals of the temperature sensors 5 . 6 bandpass filtered, which corresponds to a low-pass filtered first derivative of the temperature signals, and the calculation of the correlation function and in the SCR catalyst 4 stored amount of water is in the step 14 started. In calculating the correlation function, the result of which is the monitoring criterion, the temporal temperature gradient curves upstream and downstream of the SCR catalyst are compared. In the present embodiment, the quadratic energy cross-correlation calculated according to formula (3) is used. In this case, y _{SCRUs is} the gradient of the temperature sensor 5 measured temperature signal upstream of the SCR catalyst 4 and y _SCRDs the gradient of the temperature sensor 6 measured temperature signal downstream of the SCR catalyst 4 , The calculation of the monitoring criterion becomes dependent on the temperature of the internal combustion engine calculated at startup, since the starting temperature of the internal combustion engine has an influence on the expression of the temperature effect on the SCR catalytic converter 4 Has.

In 4 the calculation of the quadratic energy cross correlation is shown. This is in the upper graph of 4 that of the temperature sensor 5 measured temperature curve t ₅ upstream of the SCR catalyst 4 (ie on the inflow side) and, in comparison, that of the temperature sensor 6 measured temperature curve t ₆ downstream of the SCR catalyst 4 (ie downstream). The temperature profile t _where represents the downstream temperature profile in the absence of SCR catalyst, where t ₆ and t _{where are determined} at the same location, the only difference is that the temperature t _{6 is} measured when the SCR catalyst is present and the Temperature t _{where is} measured when the SCR catalyst is absent. In the lower graph in 4 is the result of the quadratic energy cross correlation function as a function of time in seconds. For calculating the correlation function c _w , the gradients of the temperature signals t ₅ , t _{6 become} upstream and downstream of the SCR catalyst, respectively 4 used. For the calculation of the correlation function c _where instead of the temperature signal t ₆ downstream of the SCR catalyst, the downstream temperature signal t is _{where used} without SCR catalyst.

The previous band pass filtering of the temperature signals allows the temperature gradient to be evaluated using the correlation function. In the following step 15 It is checked if a sufficient amount of water is the SCR catalyst 4 since the engine has started. In the lower graph of the 4 the course m _{W represents} the integral amount of water since the engine started. The arrow a ₁ or the star s ₁ mark the time when the absolute minimum value of the correlation function for an exhaust system is reached 1 with SCR catalyst. The arrow a ₂ or the star s ₂ mark the point in time at which a sufficient amount of water has reached the SCR catalytic converter and, accordingly, the monitoring criterion, ie the result of the correlation function, is evaluated. Will in step 15 the method determines that not enough water has reached the SCR catalyst since the engine has started, then the previous step 14 run again.

With the following steps 16 to 19 the method for applying the calculated monitoring criterion is described. Will in step 15 found that a sufficient amount of water the SCR catalyst to assess the monitoring criterion 4 has reached, the minimum value of the result of the correlation function, which corresponds to the minimum value of the monitoring criterion, reached up to this time is determined in step 16 compared with a predeterminable threshold value v _th . If the monitoring criterion is lower than the threshold value v _th , then it can be determined in step 17 be concluded that an SCR catalyst 4 in the exhaust system 1 is installed, or that the installed SCR catalyst 4 is functional.

If the monitoring criterion exceeds the threshold value v _th within the predefinable time from the start of the engine to the time a ₂ , then the step 18 conclude that no SCR catalyst 4 in the exhaust system 1 is available. This will be in step 19 to the control unit 7 Reported and necessary follow-up reactions are controlled, especially in the case where the compliance with the NOx limits due to the defective SCR catalyst 4 no longer exists.

In 3 schematically a flow of the two methods according to a second embodiment of the invention is shown. The steps 10 to 14 of the method are identical to the steps described in the first embodiment of the method 10 to 14 and thus describe the method for calculating the monitoring criterion. In the following steps the application of the calculated method will be described. After the calculation of the monitoring criterion, and that in the SCR catalyst 4 stored amount of water in step 14 was started, in the second embodiment, the completion of the current drive cycle in step 25 of the procedure. The following will be in step 26 determines the absolute minimum of the monitoring criterion throughout the drive cycle. The following steps 16 to 19 are again identical to the steps described in the first embodiment of the method 16 to 19 , In step 16 So that will be in step 26 determined absolute minimum compared with a predeterminable threshold value v _th . In step 17 and 18 is always concluded whether an SCR catalyst 4 in the exhaust system 1 is present or not and in step 19 the possibly required message is sent to the control unit 7 ,

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102009007763 A1 [0004]

Claims (11)

Method for calculating a monitoring criterion, which is an indicator of the presence of a zeolite-containing SCR catalyst ( 4 ) in an exhaust gas line ( 1 ) of a motor vehicle, characterized in that the monitoring criterion is calculated by temporal temperature profiles upstream and downstream of the SCR catalytic converter ( 4 ). Method according to claim 1, characterized in that the value of the monitoring criterion is dependent on the amount of water which the SCR catalyst ( 4 ), is calculated. A method according to claim 1 or 2, characterized in that the value of the monitoring criterion is calculated as a function of the temperature of the internal combustion engine when starting. Method according to one of claims 1 to 3, characterized in that the monitoring criterion by means of a cross-correlation of the temporal temperature curves or the temporal temperature gradient curves upstream and downstream of the SCR catalyst ( 4 ) is calculated. Method according to one of the preceding claims, characterized in that the method comprises the following steps: a. Exam ( 11 ), whether a heating condition for the SCR catalyst ( 4 ), b. Start ( 14 ) of the calculation of the monitoring criterion and one in the SCR catalytic converter ( 4 ) and c. Exam ( 15 ), if a sufficient amount of water is the SCR catalyst ( 4 ) has reached since the engine started. Method for using the monitoring criterion calculated according to one of the preceding claims, characterized in that the method, closing for the presence of an SCR catalyst ( 4 ) in the exhaust line ( 1 ). Process according to Claim 5, characterized in that the presence of an SCR catalyst ( 4 ) in the exhaust line ( 1 ) is closed when the monitoring criterion has a value less than a threshold (v _th ). Method according to Claim 5, characterized in that the absolute minimum of the monitoring criterion is determined at the end of the driving cycle ( 26 ). A computer program adapted to perform each step of the method of any one of claims 1 to 8. A machine-readable storage medium on which a computer program according to claim 9 is stored. Electronic control unit ( 7 ) arranged to perform the method according to any one of claims 1 to 8.

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