FIELD OF THE INVENTION
- BACKGROUND INFORMATION
The present invention relates to a device for metering a urea solution.
To reduce nitrogen oxides in the exhaust gas of motor vehicles, urea solution has in the past been sprayed into the exhaust gas during catalytic reduction. Urea is broken down into carbon dioxide and ammonia by chemical reaction on a hydrolysis catalyst. Ammonia then reacts selectively with nitrogen oxides to form nitrogen and water, thus removing nitrogen oxides from the exhaust gas.
For reliable reduction of nitrogen oxides with a urea solution, various parameters are important, in particular the urea concentration in the aqueous solution.
Sensor applications known in the past for measuring the urea concentration in the fields of medicine and biology have used urease, which enzymatically and selectively breaks down urea to form ammonia. Sensors then detect the influence of the ammonia on the pH of the solution. Information regarding the urea concentration is obtainable in this way.
One disadvantage of this method of measuring the concentration of a urea solution is the instability of urease, in particular in an environment where temperatures may vary greatly. However, such temperature variations occur during use in motor vehicles, so that previous sensors according to the related art are not suitable for such an application.
- SUMMARY OF THE INVENTION
Therefore, the object of the present invention is to propose a device for metering urea solutions which may be used reliably for reduction of nitrogen oxides, even under difficult conditions, e.g., within broad temperature intervals.
Accordingly, a device according to the present invention for metering urea is characterized in that a sensor unit is provided for monitoring a physical state variable of an enzyme-free urea solution. The sensor unit here preferably includes a measuring sensor.
In this way, a measurement is possible directly on the basis of the physical properties of urea in solution without intermediate enzymatic breakdown. Accordingly, this measurement is not subject to the instabilities to which an enzyme such as urease is subject.
In an exemplary embodiment of the present invention, a measuring sensor is provided for detecting one or more electric state variables. Such a state variable may include, for example, the pH, the dielectric constant and/or the conductance of the solution. By measuring these or other electric state variables, it is possible to obtain information regarding the properties of the urea solution, e.g., its concentration. Measurement of these state variables is comparatively unproblematical and in particular it is possible to perform these measurements in situations of extreme temperature variations.
Two electrodes may be provided to detect the electric state variables, these electrodes protruding into the urea solution. By applying an electric d.c. or a.c. voltage to the electrodes, it is possible to determine directly the aforementioned electric state variables, such as the pH, the dielectric constant, and/or the conductance.
To improve the sensitivity of the measuring sensor the electrodes may be provided with a structure which increases their surface area. Such a surface area enlarging structure may be achieved, e.g., by a comb-shaped design of the electrodes, which additionally has the advantage that two electrodes designed in this way may be arranged to intermesh, so that a small distance between the two electrodes is adjustable simultaneously with a comparatively large surface area. Due to the large surface area, in particular in combination with the small distance, the test voltage and/or test current may be reduced and therefore the control and analyzing unit for a measuring sensor according to the present invention may be designed with small dimensions. A separate electrode may be provided for simultaneous determination of multiple state variables, if necessary. For example, by using such a third electrode, it is possible to determine the pH, while another state variable, e.g., the dielectric constant, is determined using the two aforementioned electrodes.
In an exemplary embodiment of the present invention, a measuring sensor is provided for detecting one or more physicomechanical state variables of the urea solution.
Such a physicomechanical state variable may be the viscosity or density, for example.
Such physicomechanical state variables may be determined in a traditional manner, e.g., by weighing the solution and/or a part of the solution or by measuring the buoyancy of a displacement body, etc. However, in an exemplary embodiment the physicomechanical state variable is detected by a dynamic sensor. Thus, a physicomechanical state variable may be measured with the help of a vibration generator, for example. The behavior of the urea solution when agitated with the help of mechanical vibration depends to a significant extent on the physico-mechanical state variables to be detected, e.g., the density or viscosity. In an exemplary embodiment, this property may be detected directly on the vibration generator itself by measurement technology, e.g., by measuring the electric current, the frequency, etc.
A quartz oscillator may be used as the vibration generator. However, any other known or future means for inducing mechanical vibration is also conceivable. For example, a piezoelectric crystal could also be used as well as a high-speed out-of-balance motor or an electromagnetic coil in conjunction with a diaphragm based on the loudspeaker principle.
In an exemplary embodiment, a sensor unit is provided with a measuring sensor for an electric state variable and with a measuring sensor for a physicomechanical state variable. The measured values of the two measuring sensors are used in an analyzer unit to determine the urea concentration in solution. By analyzing two independent state variables, this yields the possibility of a more accurate and more selective determination of the urea concentration.
In addition, a device according to the present invention may be combined with a temperature sensor. Since the state variables to be determined may under some circumstances be dependent upon temperature, correction of errors due to temperature variations is possible through simultaneous measurement and consideration of temperature in analysis of the state variable detected, e.g., for determination of the urea concentration in solution.
In combination with a metering device for urea solution a filling level sensor may be provided for measuring the degree of filling of a storage container for the urea solution. In an exemplary embodiment, such a filling level sensor is combined directly with a measuring sensor according to the present invention for detecting one or more physical state variables.
The measuring sensor according to an exemplary embodiment of the present invention shows definite differences in the measurement in solution in comparison with the measurement in the gas phase, so a filling level may also be readily measured in this way. To do so, various embodiments of the measuring sensor according to the present invention are again conceivable. For example, a measuring sensor according to the present invention may be mounted at a certain filling level and used as a threshold value sensor as the filling level passes the threshold value. For a more precise filling level measurement at different filling levels, a plurality of sensors may also be mounted at different levels. Such a sensor system may be mounted, e.g., in a sensor housing which extends over the corresponding height or on a rod-shaped sensor mount, for example.
BRIEF DESCRIPTION OF THE DRAWING
A continuous filling level measurement may be achieved by designing the measuring sensor according to an exemplary embodiment of the present invention to extend over a corresponding height. The sensor signal here is a function of the ratio of sensor areas situated in the gas phase or in the liquid solution. These sensor areas in turn vary with the filling level, so that information about the filling level is obtainable from the sensor signal in this way.
FIG. 1 shows a schematic diagram of an exemplary embodiment of a measuring sensor according to the present invention.
Sensor unit 1 is mounted on a sensor plate 2. A comb-shaped electrode 3 is divided into two areas 4, 5. Individual teeth of the comb structure are spaced farther apart in upper area 5 than in lower area 4. In upper area 5, another electrode 6 engages with a corresponding comb structure. The two electrodes 5 and 6 extend over a large area of sensor plate 2 and constitute a filling level sensor. A third electrode 7 is situated opposite lower area 4 of electrode 3. The comb structure of electrode 7 corresponds to the finer comb structure of lower area 4 of electrode 3, i.e., the teeth are not as far apart.
Together with lower area 4 of electrode 3, electrode 7 forms a measuring sensor according to an exemplary embodiment of the present invention for measuring an electric state variable, e.g., the conductivity, the dielectric constant, etc.
Electric terminals 8 for electrodes 3, 6, 7 are provided in the lower area of sensor plate 2. These electric terminals 8 may be connected via a plug connector in a manner not shown in greater detail here.
Beneath lower area 4 of electrode 3, i.e., beneath electrode 7, a quartz oscillator 9 is shown as an oscillation generator for detecting a physicomechanical state variable, e.g., viscosity or density. Quartz oscillator 9 is also contacted via terminals 8.
In an exemplary embodiment, sensor plate 2 may be designed at least partially as a PC board on which the electrodes are implemented in the form of flat printed conductors. In exemplary embodiment, however, sensor plate 2 may function as a mounting plate for mountable electrodes.
With the help of sensor unit 1 according to FIG. 1, one or more electric state variables such as the dielectric constant, the conductivity, the pH or the like, as well as one or more physicomechanical state variables such as density or viscosity may be detected. At the same time, sensor unit 1 also functions as a filling level sensor because of the extent of upper area 5 of electrode 3 and opposing electrode 6. Sensor unit 1 is therefore mounted in the interior of a container for a urea solution, so that electrodes 3 and 6 are at least partially immersed in the urea solution.
With the help of sensor unit 1 according to the exemplary embodiment of the present invention, it is possible to reliably monitor the state of a urea solution even under adverse conditions, e.g., over a wide temperature interval. Such a sensor unit 1 is therefore suitable for use even in the area of exhaust gas processing of motor vehicles.
List of Reference Numbers:
- 1 sensor unit
- 2 sensor plate
- 3 electrode
- 4 area
- 5 area
- 6 electrode
- 7 electrode
- 8 terminals
- 9 quartz oscillator