Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/416—Systems
  • G01N27/417—Systems using cells, i.e. more than one cell and probes with solid electrolytes
  • G01N27/419—Measuring voltages or currents with a combination of oxygen pumping cells and oxygen concentration cells
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02D—CONTROLLING COMBUSTION ENGINES
  • F02D41/00—Electrical control of supply of combustible mixture or its constituents
  • F02D41/02—Circuit arrangements for generating control signals
  • F02D41/14—Introducing closed-loop corrections
  • F02D41/1438—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor
  • F02D41/1444—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the characteristics of the combustion gases
  • F02D41/146—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the characteristics of the combustion gases the characteristics being an NOx content or concentration
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02D—CONTROLLING COMBUSTION ENGINES
  • F02D41/00—Electrical control of supply of combustible mixture or its constituents
  • F02D41/02—Circuit arrangements for generating control signals
  • F02D41/14—Introducing closed-loop corrections
  • F02D41/1438—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor
  • F02D41/1473—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the regulation method
  • F02D41/1475—Regulating the air fuel ratio at a value other than stoichiometry
  • F02D41/1476—Biasing of the sensor

Definitions

• the invention is based on a method for operating a sensor element according to the preamble of the independent claim.
• Measuring gas space which is designed as a diffusion channel and in which a first and a second electrode are applied to a solid electrolyte.
• the measuring gas space is connected to the measuring gas located outside the sensor element.
• the first electrode is in the diffusion channel in
• Di fusion direction arranged behind the second electrode.
• the second electrode is coated with a layer that is impermeable to nitrogen oxides (NO x ).
• NO x nitrogen oxides
• a third electrode is provided on the side of the solid electrolyte. The first electrode and the third electrode and the second electrode and the third electrode each form a pump cell.
• a constant voltage is applied between the second and the third electrode, through which oxygen is pumped out of the diffusion channel. Because the second If the electrode is coated with a layer which is impermeable to -NO x , the NO x is not decomposed at the second electrode and can enter the gas space in the region of the first electrode.
• a constant pump voltage, the one, is also applied between the first and the third electrode
• Decomposition of the N0 X causes at the first electrode and pumps out the oxygen released by the NO ⁇ decomposition.
• the NO x concentration of the exhaust gas can be determined from the pump current between the first and the third electrode.
• DE 100 48 240 also describes a sensor element into which a measuring gas space is introduced, in which a first, NO x -storing electrode is arranged.
• the first electrode is wired so that in a first
• Time interval NO x is stored in the first electrode and that in a second time interval by applying a voltage between the first electrode and a third electrode, the NO x decomposes and the oxygen originating from the decomposition is pumped out.
• a constant oxygen partial pressure is set in the measuring gas space by means of a suitable circuit by means of a pump cell, which comprises a second electrode arranged in the measuring gas space, and a Nernst cell.
• the oxygen partial pressure is regulated to the same value during the first and the second time interval.
• the pump current that flows between the first and third electrodes and from which the NO x content of the exhaust gas is determined becomes very low at low NO x concentrations and only with ⁇ insufficient accuracy can be measured. This also limits the accuracy in determining the NO x concentration.
• the method according to the invention for operating a sensor element with the characterizing features of the first claim has the advantage over the prior art that even low concentrations of a gas component can be determined with high accuracy.
• the sensor element has a first and a second electrode arranged in a measuring gas space.
• the second electrode forms a pump cell with a third electrode arranged outside the measuring gas space, with which oxygen can be pumped into or out of the measuring gas space.
• the voltage present between the second and third electrodes is so chosen that the component of the gas to be analyzed is neither decomposed at the second electrode nor due to the equilibrium reaction taking place at low oxygen partial pressure in the measuring gas space. This ensures that the gas component to be analyzed can reach the area of the first electrode.
• a voltage which is higher than the first time interval is applied between the second and the third electrode, so that the molecular oxygen 0 2 is pumped out completely or almost completely in the measurement gas space via the first electrode.
• the oxygen partial pressure in the measurement gas space is thus lower during the second time interval than during the first time interval. This ensures that when determining the gas component to be analyzed on the first electrode, the proportion of molecular oxygen is negligible compared to the proportion of the gas component to be analyzed.
• the gas component to be analyzed can accumulate in the area of the first electrode.
• the first electrode is placed at a potential that causes a decomposition of the one to be analyzed
• Gas component causes so that the gas component to be analyzed accumulated in the region of the first electrode is decomposed.
• the concentration of the gas component to be analyzed can then be determined by passing the oxygen released by the decomposition over the first electrode is pumped out and the pump current is determined. It is also conceivable that the concentration of the gas component to be analyzed is determined by measuring the oxygen partial pressure, for example by means of a Nernst cell.
• a means for storing the gas component to be analyzed for example a storage material, is also provided in or on the first electrode or in the area of the first electrode, the gas component to be analyzed that reaches the area of the first electrode during the first time interval can be controlled in the storage material be included. If the oxygen partial pressure is reduced during the second time interval by pumping the measuring gas space via the second electrode, then a decomposition of the gas component to be analyzed stored in this material is avoided, for example by the low oxygen partial pressure or by contact with the second electrode. This ensures that the entire gas component to be analyzed that has been collected in the storage material during the first time interval can be decomposed in the second time interval. This means that even low concentrations of the gas component to be analyzed can be determined. In addition, it is ensured that contributions to the measurement signal that do not originate from pumping out the oxygen originating from the decomposition of the gas component to be analyzed are negligible.
• a means for storing the gas component to be analyzed is also to be understood as a material in which the gas component to be analyzed is stored, for example by chemisorption, in the form of a chemical compound which at least partially contains the gas component to be analyzed.
• the pump voltage between the second and third electrodes is advantageously selected so that limit current conditions are achieved. Limit current conditions are present when at least approximately all of the molecular oxygen reaching the region of the first electrode is pumped out, so that an increase in the pump voltage does not cause any or only an insignificant increase in the pump current, since the pump current only depends on the geometry of the sensor element , especially due to the diffusion resistance, restricted inflow of the relevant gas components.
• the oxygen partial pressure in the sample gas chamber can be set reliably and independently of the oxygen partial pressure in the exhaust gas.
• the second electrode is preferably arranged in such a way that it is in contact with a region of the measurement gas space located between the diffusion resistor and the first electrode.
• the oxygen diffusing from the exhaust gas into the measurement gas space can only reach the first electrode via the measurement gas space in the region of the second electrode. This ensures that the oxygen diffusing into the measuring gas space can be pumped out via the second electrode before reaching the first electrode.
• the gas component to be analyzed can be, for example, N0 X , the solid electrolyte Zr0 2 doped with Y 2 0 3 .
• N0 X the solid electrolyte Zr0 2 doped with Y 2 0 3 .
• Storage have oxides of the fifth subgroup, for example V 2 0 5 , or a mixture of oxides of the fifth subgroup, as well as barium, cerium or magnesium in the form of nitrates, oxides or . Carbonates or a mixture of the compounds mentioned have been found to be suitable.
• the process of storing the gas component to be analyzed during the first time interval and determining the gas component to be analyzed during the second time interval can be effectively supported if a temperature control at the first electrode during the first time interval results in a lower temperature than during the second time interval, since at lower temperatures, for example below 550 degrees Celsius, the NO x is stored particularly effectively, in particular in the form of nitrates.
• FIG. 1 shows a longitudinal section of a sensor element that is operated using the method according to the invention
• FIG. 2 shows a sectional illustration of the sensor element according to lines II-II in FIG. 1
• FIGS. 3a to 3d show schematic representations of the time course of the in one embodiment of the method according to the invention Operation of the sensor elements occurring electrical voltages and currents.
• FIG. 1 and FIG. 2 show a section of a sensor element 10 which is operated using the method according to the invention.
• the sensor element 10 has a first, second, third and fourth solid electrolyte layer 21, 22, 23, 24.
• a measuring gas chamber 35 is introduced into the second solid electrolyte layer 22 and is connected to an exhaust gas located outside the sensor element 10.
• the exhaust gas can enter the first solid electrolyte layer 21 introduced gas access opening 37 and a diffusion resistor 34 enter the measuring gas space 35.
• An annular first electrode 31 with a feed line 31a and an annular second electrode 32 with a feed line 32a are provided in the measurement gas chamber 35, the second electrode 32 being arranged between the hollow cylindrical diffusion resistor 34 and the first electrode 31.
• the lead 32a of the second electrode 32 is electrically insulated from the first electrode 31 by an insulation layer (not shown).
• a third electrode 33 is applied with a feed line, not shown.
• the third electrode 33 can be a porous, not shown
• a heating device 41 is provided between the third and fourth solid electrolyte layers 23, 24 for heating the sensor element.
• the first, second and third electrodes 31, 32, 33 have platinum and a Zr0 2 component as a support structure and are porous.
• the solid electrolyte layers 21, 22, 23, 24 contain Zr0 2 doped with Y 2 0 3 .
• the first electrode 31 also has a material that stores NO x .
• An oxide of the fifth subgroup, in particular V 2 0 5 , or a mixture of oxides of the fifth subgroup is suitable for this.
• the NO x storing material can consist of barium and / or cerium and / or magnesium in the form of nitrates, oxides or carbonates.
• the NO x storage material can be distributed uniformly in the first electrode 31 or can be arranged on or in the first electrode 31 as an additional porous layer.
• the first and third electrodes 31, 33 and the region of the first solid electrolyte layer 21 arranged between the two electrodes 31, 33 form a first pump cell.
• the second and third electrodes 32, 33 and the region of the first solid electrolyte layer 21 arranged between the two electrodes 32, 33 form a second pump cell.
• FIGS. 3a and 3b the profile of pump voltage u 32 and pump current I 32 of the second pump cell is shown, in FIGS. 3c and 3d the profile of pump voltage U 3 ⁇ and pump current I 3 ⁇ of the first
• a pump voltage of 0.2 V is applied to the second pump cell, which leads to a pump current I 0 , so that oxygen is pumped out of the measuring gas space 35.
• the oxygen partial pressure in the measuring gas space 35 is then generally, that is to say at the oxygen partial pressures which usually occur in the exhaust gas, above 10 -3 bar, so that at the temperatures which usually occur, NO x decomposition due to an equilibrium reaction due to an oxygen partial pressure of 'below 2 -10 "4 bar does not occur.
• NO x can reach the first electrode 31. No voltage is present at the first pump cell during the first time interval, so that the NO x accumulates in the NO x -storing material.
• the voltage at the second pump cell is increased to 1.4 V.
• the voltage can increase suddenly or extend over a certain time interval.
• limit current conditions are reached at a time t 2 lying in the second time interval, in which a pump current I 2 flows and in which the oxygen partial pressure in the measuring gas space 35 is less than 2-10 "30 bar at the oxygen partial pressures usually occurring in the exhaust gas ( at 700 degrees Celsius).
• the pump current can briefly rise to a value greater than I 2 , since the molecular oxygen present in the measuring gas space 35 is pumped out.
• a voltage of approximately 1.4 V is now applied to the first pump cell, by means of which the NO : stored in the first electrode 31 is decomposed.
• the oxygen released during the decomposition is pumped out via the first pump cell.
• the NO x concentration in the exhaust gas can be determined from the pump current flowing here.
• the molecular oxygen coming from the exhaust gas is almost completely pumped out by the second pump cell in the second time interval and therefore only makes a negligible contribution to the pump current of the first pump cell.
• the sensor element 10, in particular in the region of the first electrode 31, can be heated to a temperature in the range from 400 to by the heating device 41 during the first time interval
• 600 degrees Celsius preferably 500 degrees Celsius
• a temperature of 600 to 900 degrees Celsius preferably 780 to 850 degrees Celsius, for example 800 degrees Celsius.
• the NO x concentration can be determined, for example, by integrating the pump current flowing in the second time interval or by determining the maximum current I max flowing in the second time interval.
• the duration of the first time interval in the exemplary embodiment described is in the range from 0.2 to 20 seconds, preferably 2 seconds, the duration of the second time interval in the range from 0.1 to 2 seconds, preferably 1 second. Limit current conditions are typically reached at the latest 0.5 seconds after the start of the second time interval.
• the first time interval preferably lasts 1 second and the second time interval 0.5 seconds.
• a fourth electrode is provided which is electrically connected to the first electrode via a solid electrolyte and forms an electrochemical cell.
• the fourth electrode can, for example, like the third
• Electrode 33 can be arranged on an outer surface of sensor element 10 or in a reference gas space. If the fourth electrode is arranged in a reference gas space, the first electrode 31, the fourth electrode and one arranged between these two electrodes can
• Solid electrolyte operated by an external circuit as a Nernst cell.
• the oxygen released by the decomposition directly at the first electrode 31 provides a signal from which the NO x concentration can be determined.
• the inventive method is not only suitable for detecting the concentration of N0 X. It can also be used to detect, for example, C0 2 or S0 2 using the same storage materials.

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• 2001
  • 2001-05-04 DE DE10121771A patent/DE10121771C2/de not_active Expired - Fee Related
• 2002
  • 2002-05-02 US US10/311,945 patent/US20030164023A1/en not_active Abandoned
  • 2002-05-02 WO PCT/DE2002/001584 patent/WO2002090967A1/de not_active Application Discontinuation
  • 2002-05-02 JP JP2002588178A patent/JP2004519694A/ja not_active Abandoned
  • 2002-05-02 EP EP02737827A patent/EP1397674A1/de not_active Withdrawn

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