KR100865597B1 - Method and device for determining the level of a liquid in a container - Google Patents

Abstract

The present invention relates to a method and apparatus for measuring the filling level inside a container. It is an object of the present invention to avoid the need to continuously measure the liquid level inside the container 10 over the entire area in order to provide a clear indication of the critical liquid level. The electrodes 141, 142, 143 for liquid level measurement are formed in such a way that the measured value changes abruptly when a predetermined limit value of the liquid level is exceeded or falls short. Such abrupt changes in the measurement can be reliably detected without the need for high accuracy in the measurement. These thresholds are defined as "full container" (FS2), "minimum fill level inside container" (FS3), and "empty container" (FS4).

Description

METHOD AND DEVICE FOR DETERMINING THE LEVEL OF A LIQUID IN A CONTAINER

The present invention relates to a method and an apparatus for measuring the level of a reducing agent stored in a liquid, in particular a container, and conveyed together in an automobile for exhaust gas aftertreatment of an internal combustion engine.

Selective Catalytic Reduction (SCR) techniques for the formation of atmospheric nitrogen (N ₂ ) and water vapor (H ₂ O) can be used to reduce nitrogen oxide emissions from internal combustion engines operating in excess air, especially diesel-internal combustion engines. . As the reducing agent, for example, gaseous ammonia (NH ₃ ), an aqueous ammonia solution or an aqueous urea solution may be used. In this case urea is used as a carrier for ammonia and is injected into the exhaust system in front of the hydrolysis catalytic converter using a metering system, which is converted into ammonia by hydrolysis in the exhaust system, which is then often a NOx removal catalyst Nitrogen oxides are reduced again in a native SCR-catalyst converter, also referred to as a converter.

Such exhaust aftertreatment systems, operated with liquid reducing agents, comprise, as main components, a reducing agent vessel, a pump, a pressure regulator, a pressure sensor, a metering valve and an essential connection line. The pump transfers the reducing agent stored in the reducing agent container to the metering valve, by which the reducing agent is injected into the exhaust stream upstream of the hydrolysis catalytic converter. It is known from DE 197 43 337 C1 that the metering valve is actuated by a signal from the control device in such a way that the required amount of reducing agent is supplied according to the operating parameters of the internal combustion engine.

There is a need to reliably monitor the liquid level inside the reducing agent vessel to ensure the continuous availability of this SCR exhaust aftertreatment system. If the liquid level is lower than a predetermined value, it should be visually and / or audibly informed to the motorist that, for example, the container must be refilled at the next refueling station. In various applications, it should also be possible to infer the consumption of reducing agent from variations in liquid level to achieve improved control of the SCR method or check of the associated metering system.

Conventional liquid level sensors systems, including floats and potentiometers commonly used in fuel tanks, are not well suited for use in aqueous urea solutions due to liquid conductivity, corrosiveness, and crystallization upon complete depletion. have.

In existing systems, liquid levels are measured by measuring the electrical resistance between two good conductor electrodes (special steel rods). The electrical resistance results from the limited interelectrode conductivity of the reducing agent solution. Thus, the electrical resistance is usually inversely proportional to the depth of impregnation of the electrode. Since the conductivity of the reducing agent solution depends on the concentration, temperature and chemical composition (the amount of free ammonia contained in the solution when using the urea aqueous solution), the conductivity is determined from the ratio of the measured value of the reference electrode to that of the liquid level electrode. In order to calculate the charge level, it is additionally measured by a reference electrode independent of the charge level. It is known from DE 198 41 770 A1 that the relatively large change range of the conductivity requires a large measuring range in the electronic evaluation device, so that the measurement resolution and the measurement accuracy are limited.

It is an object of the present invention to provide a method and apparatus that facilitates the detection of the level of a conductive liquid inside a container.

This object is achieved by the features of claim 1 in the method and by the features of claim 10 in the apparatus. Preferred refinements are given in the dependent claims.

Despite the limited accuracy of the conventional sensor principle, it is not necessary to continuously measure the liquid level inside the vessel over the full range in order to be able to reliably mark critical filling level points. Rather, the electrode for level measurement is formed so that the measured value changes abruptly when it exceeds or falls short of a predetermined limit value of the liquid level. The accuracy of the measurement does not have to be so high to detect such sudden changes in the measured value. These limits include conditions such as "full container", "minimum fill level" and "empty container" as appropriate.

This gives the advantage that a simple design of the sensor electrode is sufficient and that high accuracy on the electrode surface does not need to be required. Due to the high measurement accuracy at a given point, the metered amount of reducing agent can be automatically calibrated.

If it is possible to ensure that the liquid level at the refill of the reducing agent reaches the state of the "full vessel", which is the limit value, even continuous with higher accuracy than by continuous measurement by systems known to date. Level indication can be achieved.

The approach described combines the advantages of the accuracy of limit switches with the advantages of continuous level indication. For this purpose, a mechanically simple conductivity measurement principle is combined with the possibilities provided by the control computer inside the weighing controller, allowing accurate and continuous level display, and also the automatic calibration of the reducing agent metering due to the absolutely stable limit value detection. Can be achieved.

The invention is explained in more detail below with reference to the drawings.

1 is a block diagram of an internal combustion engine with an exhaust gas aftertreatment system in which an apparatus and method for measuring liquid levels are used.

2 is a schematic view of a container having a device according to the invention.

1 shows a very simple block diagram of an internal combustion engine having an exhaust gas aftertreatment system and operated by excess air. In this drawing, only parts necessary for understanding the present invention are shown. In particular, the fuel circuit is omitted. In this embodiment, a diesel internal combustion engine is shown as an internal combustion engine, and an aqueous urea solution is used as a reducing agent for post-treatment of exhaust gas.

The internal combustion engine 1 is supplied with the air necessary for combustion through the suction line 2. An injection system, denoted by reference numeral 3, for example, which can be formed as a common rail with injection valves for directly injecting fuel KST into the cylinder of the internal combustion engine 1. The exhaust gas of the internal combustion engine 1 flows through the exhaust gas line 4 toward the exhaust gas aftertreatment system 3 and is discharged from the exhaust gas aftertreatment system 3 through the muffler (not shown) to the atmosphere. do.

For control and regulation of the internal combustion engine 1, an already known engine control device 6 is connected with the internal combustion engine 1 via a data line and a control line 7 (shown here only schematically). Signals of sensors (eg intake air, charge air, temperature sensor for coolant, load sensor, speed sensor) between the internal combustion engine 1 and the engine control device 6 via the data line and the control line 7 and Signals for actuators (eg injection valves, actuation members) are transmitted.

The exhaust aftertreatment system 5 has a reduction catalytic converter 8 comprising a plurality of catalytic converter units (not shown in detail) connected in series. One oxidation catalytic converter (not shown) may be additionally arranged upstream and / or downstream of the reduction catalytic converter 8. In addition, for the supply of the reducing agent, there is provided a metering controller 9 assigned to a reducing agent storage vessel (hereinafter simply referred to as vessel 10) with an electrically driven reducing agent pump 11. The reducing agent pump 11 may also be disposed in the vessel 10.

In this embodiment, an aqueous solution of urea stored inside the vessel 10 is used as the reducing agent. The vessel 10 has an electric heating device 12 and sensors 13, 14, which detect the temperature of the urea solution and the liquid level inside the vessel 10. In addition, signals from a temperature sensor disposed upstream of the reduction catalytic converter 8 and an exhaust gas measurement sensor, such as a NO _X sensor (not shown), disposed downstream of the reduction catalytic converter 8 are sent to the metering controller 9. Is sent.

The metering controller 9 operates an electromagnetic metering valve 15, in which an aqueous urea solution, if necessary, draws the supply line 16 from the vessel 10 using a reducing agent pump 11. Supplied through. A pressure sensor 18 is inserted into the supply line 16, which detects the pressure inside the metering system and sends a corresponding signal to the metering controller 9. By means of the metering valve 15 an aqueous urea solution is injected into the exhaust gas line 4 upstream of the reduction catalytic converter 8.

In operation of the internal combustion engine 1 exhaust gas flows through the exhaust gas line 4 in the direction of the arrow shown.

The metering controller 9 is connected to the engine control device 6 via an electric bus system 17 for mutual transmission of data. Relevant parameters for calculating the amount to be metered of the urea solution, such as engine speed, air volume, fuel volume, control path of the injection pump, exhaust gas measurement flow, operating temperature, load air temperature, injection start information, etc. Is transmitted to the weighing controller 9.

The metering controller 9 calculates the amount to be injected of the urea solution from these parameters and the measured values for the exhaust gas temperature and the NO _X content and sends the corresponding electrical signal via an electrical connection line (not shown in detail) to the metering valve. Send to (15). Urea is hydrolyzed and mixed by injection into the exhaust gas line 4. NO _x contained in the exhaust gas in the catalytic converter unit can be catalytically reduced to N ₂ and H ₂ O.

The metering valve 15 for injecting the urea solution into the exhaust gas line 4 is generally similar to a conventional low pressure benzine injection valve, which valve is for example permanently connected to the wall of the exhaust gas line 4. It is separably fixed within.

2 schematically shows a container 10 for the storage of an aqueous reducing agent 19 such as urea solution, for example, in which only the parts necessary for measuring the liquid level are shown. In particular, the inlet and outlet for the reducing agent 19, the reducing agent pump, filters and associated connection lines used for the reducing agent supply are not shown.

The container 10 is made of mainly non-conductive material or non-conductive material such as, for example, plastic and is installed to allow the driver free access inside the vehicle, or the driver only has access to the filling opening of the container 10. It is installed so that. If the container 10 is installed at a point where at least one sidewall is visible within the motor vehicle, it is desirable to select the container 10 material as a transparent material to enable further visual monitoring of the liquid level.

The vessel 10 may also be made of metal, such as aluminum. In this case, however, it must be ensured that the wall of the vessel 10 does not have a too large influence on the liquid level measurement as an additional ground potential.

A carrier part 101 is arranged on the upper surface of the container 10, which is in particular detachablely fixed to the container 10 and fixed and mutually insulated of the electrodes used for liquid level measurement. Used for The electrodes are a liquid level electrode 141, a reference electrode 142, and a common reference electrode 143, respectively.

The electrodes 141, 142, 143 are made of the same material, such as special steel or conductive plastic material, which is sufficiently high in conductivity and resistant to reducing agents. It should only be ensured that the electrical resistance of the electrodes 141, 142, 143 is much lower than the electrical resistance of the reducing agent between the two electrodes used for the measurement.

The electrodes 141, 142, and 143 each have a rod-like shape with the same cross section, but have different lengths inside the container 10. The electrodes 141, 142, 143, starting from the carrier portion 101, extend parallel to each other within the vessel 10, respectively.

The liquid level electrode 141 is covered with an electrically insulating material 1411 over most of its length. The upper region 1412 facing towards the carrier portion 101 and the end region 1413 facing towards the bottom of the container are not provided with such insulating material 1411, so that within the regions 1412, 1413 the corresponding Electrical contact with the reducing agent can be made at the liquid level.

The reference electrode 142 is likewise covered with an electrically insulating material 1411 starting from the carrier portion 101 and insulated until it reaches a point near the bottom of the container 10 in which the electrically insulating separating member 144 is disposed. The free end of the reference electrode 142, which extends to the bottom of the separating member 144 while protruding into the state, is non-insulating, thus reducing the reducing agent until the charge level FS4 falls below its corresponding low level. Electrical contact with 19 is possible.

The electrically insulating material 1411 for the charge level electrode 141 and the reference electrode 142 can be formed, for example, as an insulated hose, insulated tube, or the electrodes 141 and 142 are coated with a suitable material or subjected to injection molding. Can be coated through. In addition, the two electrodes 141 and 142 may be formed as a hollow electrode in the form of a so-called tube, and the hollow electrode simultaneously performs the functions of a supply line and a discharge line for reducing agent known from DE 198 42 484 A1, for example. .

The separating member 144 divides the entire space of the container 10 into two subspaces, wherein the space formed between the container bottom, the container side wall, and the separating member 144 and the reference measurement is made is the separating wall 144. It is much smaller than the space formed between the container side wall and the container top surface. The separating member 144 may be formed of a plate or a disk as shown in FIG. 2, and the plate or disk may have a bottom area to prevent mutual influence of the liquid level electrode 141 and the reference electrode 142. Are matched to the structure of the vessel 10 in each. In the case where the separating member 144 is extensively formed as shown in FIG. 2, it should be ensured that sufficient reducing agent can reach the space under the separating wall 144 through a recess or through hole. Within this widely separated space may be arranged a reducing agent pump and a filter or screen connected upstream of the reducing agent pump.

A common reference electrode 143 (ground electrode) is disposed between the liquid level electrode 141 and the reference electrode 142, and the common reference electrode 143 is disposed on the liquid level electrode 141 and the reference electrode 142. Is a common corresponding electrode. The common reference electrode 143 does not contain any electrically insulating material over its entire length and penetrates through the separating member 144 to the bottom of the container 10. This common reference electrode 143 is in particular placed at the ground potential of the metering controller 9 by means of a terminal. Terminals (not shown in detail) of the liquid level electrode 141 and the reference electrode 142 are connected with an input of the metering controller 9.

For the liquid level measurement, the electrical resistance between the liquid level electrode 141 and the common reference electrode 143 on the one hand and the electrical resistance between the reference electrode 142 and the common reference electrode 143 on the one hand are measured and these values are measured. The relationship between is displayed.

In order to omit the electrolytic process in the reducing agent 19, this measurement is carried out using alternating current without a direct current component. In the following description of the measurement, the electrical resistance between the common reference electrode 143 and the charge level electrode 141 (hereinafter referred to as measurement resistance R _mess ) and between the common reference electrode 143 and the reference electrode 142 It is assumed that the resistors (hereinafter referred to as reference resistors R _ref ) are identical. This is because an upper region 1412 having no insulator, a lower end region 1413 of the liquid level electrode 141, and a lower portion of the reference electrode 142 when the cross sections of all three electrodes 141, 142, and 143 are the same. Regions can be achieved by having the same length each. As a result, the same value appears for both surfaces and the following states can be distinguished in relation to the liquid level:

In the state where the vessel 10 is filled with the reducing agent 19, that is, in the liquid level “FS2”, the upper region 1412 of the liquid level electrode 141 and the lower region 1413 of the liquid level electrode 141 both contain a reducing agent ( 19) impregnated into This allows two identical electrical resistances to be connected in parallel according to the selected structural shape of the electrode regions 1412 and 1413 impregnated into the reducing agent 19, and the lower region 1421 of the reference electrode 142 when the vessel 10 is fully filled. ) Is also impregnated into the reducing agent 19, so that the value of the measurement resistance R _mess is half of the value of the reference resistance R _ref .

If the liquid level "FS1" in the vessel 10 is located below the upper region 1412 of the liquid level electrode 141 but is still above the lower region 1413 of the liquid level electrode 141, two The measured values R _mess and R _ref are identical. When the reducing agent in the vessel 10 falls below the region 1413 of the liquid level electrode 141, that is, when the two regions 1412 and 1413 are not wetted with the reducing agent, the electrical resistance measured is determined by the insulation resistance ( Ideally high enough to be infinitely large resistance). This charge level is referred to as the minimum charge level FS3.

The value of the minimum fill level FS3, and the distance between the bottom end of the region 1413 and the bottom of the vessel 10, is the next time the fuel needs to be filled more when the minimum fill level FS3 inside the vessel 10 is reached. By refueling, the amount of reducing agent 19 is sufficient to allow the vehicle to be operated by the exhaust aftertreatment system, even if the vehicle is not fueled just before the minimum fill level (FS3) is reached. 10) It is determined to exist inside.

If the reducing agent 19 is not refilled, due to the metering of the reducing agent 19, the level inside the vessel 10 will no longer impregnate the lower region 1421 of the reference electrode 142 into the reducing agent 19 at a given point in time. Keep falling until not (liquid level FS4). This state can also be easily detected, because in this case, not only the electrical resistance R _mess at the charge level electrode 141 but also the electrical resistance R _ref at the reference electrode 142 is very high ( Insulation resistance, ideally infinitely large electrical resistance).

In some cases, the liquid film or liquid droplets remaining on the electrode do not adversely affect the measurement. This is because a thin film or liquid droplet formed on the surface in the region of the electrode substantially does not contribute to the conductivity as compared to the state impregnated in the reducing agent.

The comparison and evaluation of the values R _mess and R _ref is made in a manner known in the weighing controller 9. Three liquid levels, FS2 (full container), FS3 (lower minimum fill level), and FS4 (empty container), which are important for motorists, are especially visually displayed on the instrument cluster.

If the vehicle is provided with a continuous liquid level indication function for the liquid reducing agent 19 stored in the container 10, the device is preferably used in a complementary form for the liquid level measurement. The continuous liquid level indication can be made for example on the basis of the calculated value of the metering controller 9. Since a predetermined amount of reducing agent 19 is injected into the hydrolysis catalytic converter per each metering pulse, the current liquid level inside the vessel 10 can be deduced by adding the number of metering pulses to the known starting amount of the supplied reducing agent. have. Therefore, it should be ensured that the vessel 10 is fully filled with reducing agent at the beginning of the calculation of the current liquid level.

The following procedures are allowed by combining this continuous liquid level indication with the described three-point liquid level measurement:

If the container 10 is installed at an inaccessible point in the motor vehicle and the filling of the container is via a pump, the liquid level “FS2” is reached and then the metering controller 9 shuts off the pump so that the container 10 is closed. The reducing agent 19 flows into the vessel 10 until it is fully filled and the filling is stopped. The continuous liquid level display will now display 100% or the maximum filling amount, for example in liters. During driving of the motor vehicle the metering controller 9 continuously sums up the metered amount of the reducing agent and calculates the current liquid level from it. When the current liquid level reaches the limit "minimum fill level" (liquid level FS3), a signal is issued by operating a warning device to inform the motorist that the reductant must also be recharged at the next gas station. In addition, the metering controller 9 can compare its calculated value with the actual liquid level and in some cases correct the data used for the throughput of the metering valve 15. Weighing to prevent dry running of the weighing system if the reducing agent 19 is not refilled at the next refueling or if a special situation has previously reached the limit "empty container" (liquid level FS4). The stop of the pump stops the metering of the reducing agent.

Claims (20)

As a method for measuring the level of a conductive liquid 19 inside a vessel 10, in particular a urea solution in a urea reservoir, Measuring the electrical resistance R _mess between the liquid level electrode 141 and the common reference electrode 143, Measuring an electrical resistance R _ref between the reference electrode 142 and the common reference electrode 143, _Deduce the liquid level inside the vessel 10 from the values of the electrical resistances R _mess , R _ref , _Separately detecting an electrical resistance R _mess between the liquid level electrode 141 and the common reference electrode 143 and an electrical resistance R _ref between the reference electrode 142 and the common reference electrode 143, respectively. by doing, The individual value of the electrical resistance R _mess , R _ref varies greatly when the at least one predetermined limit value FS2, FS3, FS4 for the liquid level is exceeded or falls short, How to measure liquid level. The method of claim 1, The over or under conditions of the thresholds FS2, FS3, FS4 are visually displayed to the driver, How to measure liquid level. The method of claim 1, As the threshold value, a liquid level FS2 is selected which indicates a state in which the container 10 is completely filled with liquid 19, How to measure liquid level. The method according to any one of claims 1 to 3, As the threshold value, a liquid level FS3 is selected which indicates that the liquid 19 inside the container 10 is at the minimum fill level, How to measure liquid level. The method according to any one of claims 1 to 3, As the threshold value, the liquid level FS4 representing the empty container 10 is selected, How to measure liquid level. The method of claim 2, When the liquid level “FS2” is reached, the pump used to fill the container 10 with liquid 19 is stopped. How to measure liquid level. The method of claim 3, wherein If the limit value FS3 is not reached, the warning device is activated. How to measure liquid level. The method of claim 4, wherein If the liquid level FS4 is not reached, the operation of the metering pump 11 used for metering the liquid 19 is stopped. How to measure liquid level. The method of claim 2, When it is detected that the container 10 is in a fully filled state, this state is used as a reference value for the continuous liquid level indication provided in the vehicle. How to measure liquid level. A conductive liquid 19 inside the container 10, in particular an element in the element reservoir, comprising a conductive liquid level electrode 141, a reference electrode 142 and a common reference electrode 143 disposed inside the container 10. An apparatus for measuring the level of a solution, The electrical resistance R _mess between the liquid level electrode 141 and the common reference electrode 143 and the electrical resistance R _ref between the reference electrode 142 and the common reference electrode 143 are measured to allow the container ( 10) used as a reference for the liquid level inside, The liquid level electrode 141 and the reference electrode 142 are partially covered with the insulating material 1411, thereby selecting regions 1412, 1413, corresponding to the liquid levels FS2, FS3, FS4, respectively, respectively detected. Only 1421 has electrical contact with the liquid 19, thereby _reducing the electrical resistance R _mess , R _ref when the liquid 19 reaches or leaves the regions 1412, 1413, 1421. The value changes significantly, Liquid level measuring device. The method of claim 10, The liquid level electrode 141 is substantially covered with an insulating material 1411 along its length extension, the top region 1412 facing the top surface of the vessel 10 and the bottom of the vessel 10. In the lower region 1413 facing the side, it does not include the insulating material 1411, Liquid level measuring device. The method of claim 11, The lower region 1413 is in electrical contact with the liquid 19 until the axial length of the liquid level electrode 141 falls below the minimum fill level FS3 based on the height of the vessel 10. Chosen to be, Liquid level measuring device. The method of claim 11, The position of the upper region 1412 is selected such that the upper region 1412 is not in electrical contact with the liquid 19 unless the maximum fill level FS2 is reached. Liquid level measuring device. The method of claim 10, The reference electrode 142 is substantially covered with insulating material 1411 along its length extension and does not include the insulating material 1411 in the lower region 1421 facing the bottom of the container 10. not, Liquid level measuring device. The method of claim 14, Based on the height of the vessel 10, the axial length of the reference electrode 142 may cause the lower region 1421 to reach the liquid 19 until it reaches a liquid level FS3 representing the empty vessel 10. Electrical contact with) Liquid level measuring device. The method according to any one of claims 10 to 15, The space of the container 10 is separated into two spaces of different sizes by the electrically insulating separating member 144, and the lower region 1421 of the reference electrode 142 faces the bottom of the container 10. Set within the space, Liquid level measuring device. The method of claim 10, The axial length of the common reference electrode 143 reaches the bottom of the container 10, Liquid level measuring device. The method of claim 10, The liquid level electrode 141, the reference electrode 142, and the common reference electrode 143 are formed of an insulating material having an electrical resistance much lower than the conductivity of the liquid 19 inside the container, Liquid level measuring device. The method of claim 18, The electrodes 141, 142, 143 are made of special steel, Liquid level measuring device. The method of claim 18, The electrodes 141, 142, 143 are made of a conductive plastic, Liquid level measuring device.

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