Abstract:
A method and a device for metering a reducing agent is described, which is used for mass regulated metering of a reducing agent, in particular urea or a urea-water solution for the exhaust gas treatment of the exhaust gas of a diesel engine in particular. The device includes a mass sensor for measuring the reducing agent mass flow into the catalytic converter system.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to a method and a device for metering a reducing agent, in particular urea, or a urea-water solution, within the scope of a catalytic exhaust gas treatment. 
   BACKGROUND INFORMATION 
   In order to achieve a reduction of NO x  components in exhaust gases, reduction-type catalytic converters have been developed, in particular for diesel engines, which are commonly subdivided into SCR catalytic converters (“selective catalytic reduction”) and storage-type catalytic converters. SCR converters are regenerated by supplying a urea-based reducing agent and/or an ammonia-based reducing agent, while storage-type converters are regenerated during rich exhaust phases by using the entrained hydrocarbons of the engine fuel. 
   A device, which meters urea as a reducing agent for the removal of nitrogen oxides from exhaust gases, e.g., from a diesel engine, is known from German Published Patent Application No. 199 46 900. Metering takes place by using a valve which lets through doses of urea which are determined by the electrical control of the metering valve, its throttle cross section, and the pressure difference applied to the throttle valve. The pressure upstream from the valve is measured and is kept constant within a tolerance range. 
   SUMMARY OF THE INVENTION 
   The method according to the present invention and the metering device according to the present invention have the advantage over the related art in that they lower the metering tolerances to values below ±10%, e.g., to values of approximately ±5%. Series scattering, in particular of the throttle cross section, due to manufacturing tolerances of the bore diameter, as well as the inflow and outflow edges, may be compensated; temperature dependencies in the dosage, e.g., in the temperature range of −10° to +100° C., due to the temperature-dependent viscosity of the fluid or due to the change in length of the components as a result of temperature changes, may also be compensated. Also time drifts over the entire service life of the metering device (up to 10,000 hours of operation) may be compensated. By determining the mass, it is possible to provide a closed-loop control circuit and a closed-loop control method based on metering by detecting the metering mass flow, compiling the measured values by the control unit, and corrected control of the metering valve or of a metering pump. Thus it is possible to correct the actual values in the mass flow with the purpose of adjusting them to the setpoint values, thereby improving the metering accuracy. In addition, changes, influenced over time, may be detected and corrected. The metering tolerance is then only influenced by the measuring tolerance of the mass sensor, resulting in a substantial reduction in the number of components that influence the tolerance. 
   It is in particular advantageous if an essentially known mass sensor is used which, in addition to the mass flow, measures the density of the medium flowing through so that the working substance used and also the physical state(s) of the working substance(s) may be identified. By using an aqueous urea solution it is possible to decide on the density and concentration of the urea. Changes in the concentration may thus be compensated within specified limits by modified control of the metering valve. The metering accuracy is thereby further improved and, in the event of deviations from the normal concentration exceeding the tolerance value, an error message may be triggered via a warning display optionally connected to the control unit. Mistakes in application, e.g., the use of wrong working substances, e.g., water without a urea additive, or methanol, or fuel, may be detected and this information may be used for emergency shutdowns of the system or for error messages. Also phase changes in the reducing agent may be detected, e.g., the formation of vapor bubbles after degassing, ice formation after freezing, and a formation of air bubbles. The control unit may initiate appropriate venting and heating procedures, and a return line to the tank or a separate bleeder valve may be omitted. 
   The metered supply of the reducing agent is implemented in a particularly simple manner by using an appropriate electrically controllable metering pump, since a pump unit for the transportation of the urea is necessary anyway. 
   Furthermore, it is advantageous to provide an electrically controllable metering valve which may be electrically controlled by a control unit which in turn ensures mass-regulated metering, either alone or in combination with an appropriate control of the metering pump. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a metering device which is connected to a water tank, a urea tank, and a catalytic converter system. 
       FIG. 2  shows a corresponding system using an alternative metering device. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1  a urea tank is labeled  1  from which a urea-water solution is aspirated by a metering pump  4  via urea line  1   a  including check valve  2  and filter  3 , designed as a filtering screen, and transported to a metering valve  7  of a mixing chamber  8 . For minimizing the amount of overflow, pump  4  is speed-controlled by a pilot motor  4   a . A pumped overflow is directed back to the suction side of the pump via a pressure relief valve  11 . Compressed air from a compressed air container  20  is introduceable into the mixing chamber via an air line  2   a  including a filtering screen  21 , a 2/2 directional control valve  22 , a throttle  23 , and a check valve  24 . An aerosol line  25  runs from mixing chamber  8  to catalytic converter  30  which has an exhaust gas intake  29  on one side and an exhaust gas outlet  31  on the opposite side. Urea tank  1  is provided with a filling level sensor  52  and a temperature sensor  51 . A mass sensor  50  is situated between metering pump  4  and metering valve  7 . Temperature sensors  53  and  54  measure the exhaust gas temperature at the intake and the outlet of catalytic converter  30 . In addition, a pressure sensor  55  is provided between 2/2 directional control valve  22  and throttle  23 . A temperature sensor  56  measures the temperature of a metallic housing block  41  on which components, framed by the broken line and labeled with this reference number, are situated or integrated. In addition, a control unit  40  is attached to housing block  41  and is electrically connected to sensors  50  through  56 , as well as to pilot motor  4   a  and metering valve  7 . Housing block  41  is grounded; control unit  40  uses the electrical potential of housing block  41  as the reference potential. Control unit  40  is connected to the power supply and other electrical components in the motor vehicle, the engine control unit in particular, via a CAN data line  39 . (CAN stands for “controlled area network”.) 
   Metering valve  7  meters the required urea-water solution into mixing chamber  8 . An aerosol and a wall film, generated in the mixing chamber by exposing the urea-water solution to the compressed air, are introduced into catalytic converter  30  via aerosol line  25 . Control unit  40  detects signals which are received from a master engine control unit via CAN data line  39 , as well as the signals from pressure, temperature, and filling-level sensors  51  through  56  which are known per se and are not further explained here. In addition, control unit  40  receives an electrical signal from mass sensor  50 , resulting in the time-dependent mass flow rate of the reducing agent between metering pump  4  and metering valve  7 . From the sensor information control unit  40  calculates a urea metered amount which is to be added to the exhaust gas streaming through catalytic converter  30 . By utilizing known inductive and/or mechanical methods, mass sensor  50  measures the flow rate of the reducing agent via a defined flow cross section and generates an electrical signal proportional to the mass flow. With the aid of metering valve  7  and valve  22 , control unit  40  regulates the urea-water solution pressure and the pressure in the air line. For this purpose, the control unit uses data of the engine operating state supplied by the engine control unit via data line  39 , as well as the sensor data originating in the metering device and the catalytic converter. Mass sensor  50  identifies the mass of urea-water solution flowing through and relays the measured value also to the control unit so that the actual mass flow is detected via a closed-loop control circuit, and the control of metering valve  7  may be adjusted to the setpoint mass flow. 
   The metering device may alternatively also be used without compressed air support, i.e., without the use of components  20  through  24 . The mass sensor may also be designed such that, in addition to the mass flow, it also determines the density of the medium flowing through the measuring cell. Such mass sensors are also known per se. If an aqueous urea solution is used as the flowing medium, then the concentration may be determined via the density by using such measuring elements. Changes in the concentration may be compensated within specific limits by changing the control of the metering valve, in that respective concentration correction curves are stored in control unit  40 . The metering accuracy is thereby further improved, i.e., an error message may be triggered in the event of an excessively high or an excessively low concentration. In addition, the control unit may determine whether wrong working substances are used and it is able to utilize this information for emergency shutdowns of the system and error messages. Information about the density of the medium may also be used by the control unit for the purpose of detecting phase changes, e.g., vapor bubbles after degassing, ice formation after freezing, as well as air in the event of formation of air bubbles. The control unit may then initiate appropriate venting and/or heating procedures. Venting procedures may be executed via metering valve  7 , and heating may be performed via heating elements (not illustrated) which are in thermal contact with housing block  41 . 
   A commercially available mass sensor operating on the Coriolis principle may also be used as an alternative to a volume flow measurement through a defined flow cross section. An additional alternative is a scale design similar to an essentially known fuel scale which weighs a defined volume. It is also possible to design the mass sensor similar to a hot-wire air mass sensor which determines the mass flow via the cooling down of a heated wire induced by the flowing fluid. Furthermore, independent from the specific design of the mass sensor, a temperature sensor may be integrated in it which measures the temperature of the working substance and communicates it to the control unit so that the control unit is able to execute a temperature adjustment of the mass flow sensor in order to take the temperature dependency of the kinematic viscosity of the fluid into account. 
     FIG. 2  shows a further alternative embodiment in which the same components are referenced with the same reference numbers as in  FIG. 1 . In contrast to  FIG. 1 , pressure relief valve  11  is omitted and a check valve  110  is provided instead of metering valve  7 . 
   In contrast to the system according to  FIG. 1 , this is a simplified embodiment in which metering takes place only via the metering pump.