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
  • G01N27/403—Cells and electrode assemblies
  • G01N27/406—Cells and probes with solid electrolytes
  • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
• • 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
  • G01N27/403—Cells and electrode assemblies
  • G01N27/406—Cells and probes with solid electrolytes
  • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
  • G01N27/4073—Composition or fabrication of the solid electrolyte
  • G01N27/4074—Composition or fabrication of the solid electrolyte for detection of gases other than oxygen

Definitions

• the invention relates to a method for determining the NOx concentration in the exhaust gas of an internal combustion engine according to the preamble of claim 1.
• a thick-film sensor To measure the NOx concentration in a gas, for example in the exhaust gas of an internal combustion engine, it is known to use a thick-film sensor.
• a thick-film sensor is described, for example, in the publication N. Kato et al. , "Thick Film Zr0 2 NOx Sensor for the Measurement of Low NOx Concentration ,,, Society of Automotive Engineers, publication 980170, 1989, or in N. Kato et al. , “Performance of Thick Film NOx Sensor on Diesel and Gasoline Engines ,,, Society of Automotive Engineers, publication 970858, 1997.
• This sensor has two measuring cells and consists of a zirconium oxide that conducts oxygen ions.
• a first oxygen concentration is set by means of a first oxygen ion pump current, with no decomposition of NOx taking place.
• a second measuring cell which is connected to the first via a diffusion barrier, the oxygen content is further reduced by means of a second oxygen ion pump current.
• the decomposition of NOx at a measuring electrode leads to a third oxygen ion pump current, which is a measure of the NOx concentration.
• the entire sensor is brought to an elevated temperature, for example 750 ° C., by means of an electric heater.
• This measurement of the NOx concentration results in a deviation of the true NOx concentration, since a slip of oxygen from the first into the second chamber leads to a falsification of the NOx measured value.
• Pumping out the acid Material slip in the second cell reduces the oxygen content falsifying the measurement signal considerably, but not completely, since oxygen that does not come from the decomposition of NOx is still included.
• the invention is based on the knowledge that the measurement error of the value for the NOx concentration supplied by a NOx measuring sensor constructed in a two-chamber construction depends on the level of the oxygen concentration in the exhaust gas of an internal combustion engine.
• the sensor cannot measure the oxygen concentration.
• the first oxygen ion pump current is a direct measure of the air ratio ⁇ of the exhaust gas. If one does not relate this measurement error to the
• largely linear means that higher order terms only have to be taken into account with very small coefficients.
• An air ratio signal preferably the 1 / ⁇ value of the exhaust gas, is thus obtained from the first oxygen ion pump current in the first measuring cell, by means of which the measurement error is determined using a relationship between the air ratio signal and measurement error.
• the measured NOx concentration is then corrected for this measurement error.
• the relationship is preferably determined beforehand in a calibration measurement, so that the measurement error in the form of a characteristic of, a characteristic curve or a functional relationship.
• a characteristic curve is also determined which reflects the relationship between the first oxygen ion pump current in the first cell and the air ratio signal, for example 1 / ⁇ or another function of ⁇ .
• the relationship between the first oxygen ion pumping current and 1 / ⁇ is surprisingly largely linear, so that it is further advantageous to select 1 / ⁇ as the air ratio signal.
• the method according to the invention has the advantage that only multiplications, subtractions and additions are necessary when correcting the measured NOx concentration. A division that would unduly stress the computing power of an inexpensive microcontroller is not required.
• FIG. 1 shows a schematic representation of a device for carrying out the method according to the invention
• Fig. 2 is a schematic sectional view of a NOx sensor
• FIG. 3 shows a schematic flow diagram for carrying out the method according to the invention.
• a section through a NOx sensor 1 is shown schematically.
• This sensor 1 is used in the device shown in FIG. 1 as a sensor 24 for loading Mood of the NOx concentration in the exhaust tract 27 of an internal combustion engine 20 is used.
• the measured values of the NOx sensor 24 are read out by a control unit 23, which is connected to the NOx sensor 24, and fed to an operating control device 25 of the internal combustion engine 20, which controls a fuel supply system 21 of the internal combustion engine 20 so that a NOx reducing catalytic converter 28, which in this case is located upstream of the NOx sensor 24 in the exhaust tract 27 of the internal combustion engine 20, shows optimal operating behavior.
• the sensor 24, 1 is shown in more detail in FIG. 2.
• the exhaust gas diffuses through the diffusion barrier 3 into a first measuring cell 4.
• the oxygen content in this measuring cell is measured by tapping a Nernst voltage between a first electrode 5 and a reference electrode 11 exposed to ambient air.
• the reference electrode 11 is arranged in an air duct 12 into which ambient air passes through an opening 14.
• the tapped Nernst voltage is fed to an 8-bit microcontroller which serves as controller C0 and which provides a control voltage VSO.
• controller C0 which controls a voltage-controlled current source UI0 which drives a first oxygen ion pump current IP0 through the solid electrolyte 2 of the sensor 1 between the first electrode 5 and an outer electrode 6.
• a predetermined oxygen concentration is regulated by the regulator C0 m of the first measuring cell 4 by means of the control voltage VSO. This is measured via the Nernst voltage between the electrode 5 and the reference electrode 11, so that the control loop of the controller C0 is closed.
• the first oxygen ion pump current is a measure of the air ratio in the exhaust gas, as is known from lambda probes.
• the circuit arrangement described thus sets a predetermined oxygen concentration in the first measuring cell 4.
• the second measuring cell 8 is connected to the first measuring cell 4 via a further diffusion barrier 7.
• the gas present in the first measuring cell 4 diffuses into the second measuring cell 8 through this diffusion barrier 7.
• a second oxygen concentration is set in the second measuring cell via a circuit arrangement.
• a second Nernst voltage is tapped between a second electrode 9 and the reference electrode 11 and fed to a regulator C1, which provides a second actuating voltage VS1 with which a second voltage-controlled current source Uli is controlled.
• the circuit arrangement for driving the oxygen ion pump current IP1 out of the second measuring cell 8 thus corresponds to the circuit arrangement for the first measuring cell 4.
• the circuit arrangement drives the oxygen ion pumping current IP1 in such a way that a predetermined oxygen concentration is established in the second measuring cell 8.
• This oxygen concentration is chosen so that NOx is not affected by the processes taking place, in particular no decomposition takes place.
• the NOx is now pumped at the measuring electrode 10, which can be configured catalytically, in a third oxygen ion pumping current IP2 from the measuring electrode 10 to the outer electrode 6. Since the residual oxygen content in the measuring cell 8 has been reduced sufficiently, this oxygen ion pumping current IP2 is essentially carried only by oxygen ions which originate from the decomposition of NOx at the measuring electrode 10. The pump current IP2 is therefore a measure of the NOx concentration in the measuring cell 8 and thus in the exhaust gas to be measured.
• This pump current IP2 is driven by a voltage-controlled current source UI2, the actuating voltage VS2 of which is predetermined by a regulator C2, which controls the Nernst voltage between the measuring electrode 10 and the Taps reference electrode 11 and regulates a predetermined Nernst voltage by specifying the control voltage VS2.
• the residual oxygen content of the measuring cell 8 is only ideally zero, since a slip of oxygen from the first into the second measuring cell still causes the measured NOx concentration to depend on the oxygen concentration in the exhaust gas.
• an oxygen signal 1 / LAM is obtained from the first oxygen ion pumping current IPO in the first measuring cell 4, which signal expresses the air ratio in the exhaust gas.
• the conversion of the first oxygen ion pump current IPO is carried out using a characteristic curve 15 or a characteristic map which assigns a 1 / ⁇ value to each current, which in this case is the air ratio signal 1 / LAM.
• the characteristic curve 15 was previously determined for the respective measuring sensor 1 in a calibration measurement.
• This air ratio signal 1 / LAM is implemented with a further characteristic curve 16 m, a measurement error NOx_D.
• the characteristic curve 16 was obtained from a corresponding calibration measurement of the sensor 1 and prints out the relationship between the measurement error and 1 / LAM.
• the conversion using a characteristic curve is replaced by a conversion using a functional relationship. If the 1 / ⁇ value is taken as the air ratio signal, there is a largely linear relationship. If one cannot rely on such a substantially linear relationship, the characteristic curve 16 is stored instead of a function. In the following, however, it is assumed that the 1 / ⁇ value is used as the air ratio signal 1 / LAM and thus the truncation is based on a largely linear relationship. Then through simple multiplication of the value of the air ratio signal 1 / LAM by a multiplication factor and addition of an addition factor, a measurement error N0x_D can be obtained. The corrected NOx concentration NOx_C is obtained by simply multiplying this measurement error N0x_D, which is then implemented, for example, as a correction multiplier, in arithmetic operation 17 by the measured NOx concentration NOx-M.
• the characteristic curve 15 can be combined with this characteristic curve 16, so that directly from the first Oxygen ion pump current IPO the measurement error N0x_D, for example as a multiplication or addition correction factor. Then one step is omitted because the generation of the air ratio signal 1 / LAM is dispensed with. However, if this air ratio signal 1 / LAM is required, for example for other control or regulating functions when operating the internal combustion engine, it can of course still be generated from the first oxygen ion pumping current IPO.

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• Chemical & Material Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Health & Medical Sciences (AREA)
• Physics & Mathematics (AREA)
• Chemical Kinetics & Catalysis (AREA)
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• General Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Combined Controls Of Internal Combustion Engines (AREA)
• Investigating Or Analysing Materials By The Use Of Chemical Reactions (AREA)

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• 1999
  • 1999-11-25 DE DE19956822A patent/DE19956822B4/de not_active Expired - Lifetime
• 2000
  • 2000-11-22 JP JP2001540362A patent/JP4746239B2/ja not_active Expired - Lifetime
  • 2000-11-22 EP EP00989790A patent/EP1232391A2/de not_active Withdrawn
  • 2000-11-22 WO PCT/DE2000/004128 patent/WO2001038864A2/de not_active Application Discontinuation
  • 2000-11-22 KR KR1020027006735A patent/KR100754535B1/ko active IP Right Grant
• 2002
  • 2002-05-28 US US10/156,483 patent/US6699383B2/en not_active Expired - Lifetime

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