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
• • 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
  • G01N27/4065—Circuit arrangements specially adapted therefor

Definitions

• This invention relates to determining the composition of a gaseous atmosphere.
• A/F » mass of air/mass of fuel An important application of high temperature gas sensors is in the determination of the air to fuel ratio (A/F » mass of air/mass of fuel) in the exhaust gases of hydrocarbon fired furnaces or engines such as an autorao- bile internal combustion engine.
• the stoichiometric A/F is one in which the mass of air present contains just enough oxygen (0 ) to react with the hydrocarbons (HC) present so that there is the minimum amount of both O2 and HC remaining.
• HC hydrocarbons
• the stoichiometric A/F is usually 14.6. If an engine were running lean of stoichiometry (A/F > 14.6) there is a substantial excess of oxygen in the exhaust gas which increases monotonically with A/F thereby providing a measure of the latter quantity.
• Patent 3,948,081 to Wessel et al describes a single electrochemical cell device which is convenient for 0 sensing at or near the stoichiometric A/F.
• the electrolyte is in the form of a cylinder closed at one end which is inserted into the exhaust gas. Inner and outer surfaces of the closed end are coated with thin platinum electrodes so that a cell is formed.
• the open end of the tube is exposed to a reference atmosphere (usually air) so that the 0 2 partial pressure adjacent to the inner electrode is given by PREF- P EX' tl ⁇ e °2 Partial pressure in the exhaust gas, is adjacent to the outer electrode.
• V the output voltage V of the cell is a sensor of P EX and accor ⁇ dingly of exhaust gas A/F.
• An advantage of this device is its simplicity.
• a disadvantage is its low sensitivity to P E ⁇ because of the logarithmic function.
• This disadvan- tage is offset near the stoichiometric A/F because P EX can change abruptly by more than twenty orders of magnitude within a very small A/F region near the stoichiometric value.
• V ⁇ 1.0 volt
• oxygen pumping devices can also be used to measure A/F with higher sensitivity.
• U.S. Patents 4,210,509 to Obyashi et al; 4,224,113 to Kimura et al; and 4,169,440 to Taplin et al describe single cell devices which can perform such rich A/F measurements. These measurements require the simul ⁇ taneous measurement of oxygen pump current, I p , through the cell as well as the voltage V p across the cell.
• a steady-state method for determining rich values of A/F includes generating a signal proportional to the concentrations of unreacted and partially reacted hydrocarbons. Such hydrocarbons can occur in the exhaust gases of internal combustion engines.
• the method utilizes a structure which includes a first and a second electrochemical cell spaced from one another and defining between them a partially enclosed volume.
• the volume is in communication with the exhaust gases through an opening.
• a first side of each of the first and second electrochemical cells is exposed to the volume.
• a second side of the first electrochemical cell is exposed to the exhaust gases.
• a second side of the second electrochemical cell is exposed to a reference atmosphere usually air.
• applied pumping current causes 0 2 to be pumped into the partially enclosed volume from the reference atmosphere.
• This 0 2 reacts with unreacted and partially reacted HC within the volume which in turn causes an EMF to be generated across the other electrochemical cell.
• This EMF is used to control the pumping current so that 0 2 is pumped into the volume at a rate which will keep the induced EMF fixed at an arbitrary value.
• the magnitude of the steady-state pumping current required to accomplish this task is found to be pro ⁇ portional to the concentrations of unreacted and partially reacted HC in the exhaust gas and hence is inversely proportional to rich A/F ratio thus providing a sensor of that quantity.
• Fig. 1 is a schematic cross section of a sensor structure for making rich A/F measurements in accordance with an embodiment of the steady-state pumping method of this invention
• V/ VvIP Fig. 2 is a graphic representation of sensor cell voltage, V g , versus pump cell current I p at various rich A/F values for the sensor structure shown in Fig. 1;
• Fig. 3 is a graphic representation of the pump cell current I p required to hold the voltage of the sensor cell at a reference voltage for various A/F values in accordance with the sensor structure shown in Fig. 1;
• Fig. 4 is a schematic diagram of the sensor structure shown in Fig. 1 with the addition of external circuitry for use in accordance with an embodiment of this invention.
• an air fuel sensor 110 includes an electrochemical cell 111 including a disk-like electrolyte 112 of a solid ionic conductor of oxygen such as 2O3 doped Zr ⁇ 2- Cell 111 also includes two thin porous catalytic platinum electrodes 113 with attached lead wires 114. Similarly, an electrochemical cell 121 includes an electrolyte 122, electrodes 123 and leads 124. Electrochemical cell 111 is separated from electrochemical cell 121 by a thin, generally cylindrical and hollow spacer 125 so that an enclosed volume v is defined. Cell 111 has a small hole or leak aperture 126 in it so that an ambient environment, the exhaust gas, can establish itself within the volume v.
• Electrochemical cell 121 is made in such a form, or has structure attached to it, so that electrolyte 122 has a thimble-like tubular shaped closed at one end thereby defining a reference volume and exposing one side of cell 121 to a reference atmosphere. As a result, one side of the sensor is exposed to the exhaust gas and one
• the OMPI side is exposed to the reference atmosphere.
• the sensor supporting structure 128 shown schematically provides a seal between exhaust and reference atmospheres as well as allowing for sensor attachment to the exhaust pipe wall 127 in addition to providing structural support and protection. Openings 130 in a sensor support structure cover 228 allow easy access of the exhaust gas to sensor 110. Lead wires 114 and 124 are passed through support structure 128 for attachment to external circuitry. A heater 129 is provided to keep A/F sensor 110 within a desired operating temperature range.
• the sensor structure of Fig. 1 can be used to determine rich A/F ratios in accordance with a steady-state embodiment of this invention.
• the method causes O2 to be pumped into v by cell 121 (pump cell) from the reference atmosphere at a rate given by I p . Simul ⁇ taneously, the oxygen partial pressure within v is decreased by oxygen diffusion through leak aperture 126 and chemical reaction with the partially reacted HC at interior catalytic electrodes 123 and 113.
• V s the magnitude of this EMF, termed V s , is again given by Equation (1) where PREF is replaced by P v which represents the near equilibrium oxygen partial pressure within volume v resulting from the reaction of pumped oxygen and partially reacted HC.
• Fig . 2 shows a plot of induced EMF, V s , versus pump current, I p , at different r ich air fuel ratio values.
• the EMF is low for small pump currents and increases with
• FIG. 4 A convenient circuitry for implementing this method with the structure of Fig. 1 is shown in Fig. 4.
• Resistors R ⁇ , R 2 and capacitor C control the gain and frequency response of amplifier A so that A will always generate enough pump current I p to maintain the EMF across cell 111 at a constant value equal to V(REF).
• a resistor R3 is included in the pump cell circuit so that Ip can be determined by measuring the voltage across R3 with the voltmeter V. Using the calibration curves of Fig. 3 the air fuel ratio would thus be determined. Using known electronic circuitry this current can be compared to the value of I p required for a desired air fuel ratio.
• a temperature sensor 140 which in combination with the voltage drop across R 3 , form the inputs to correction circuitry 141, to adjust I p to a temperature compensated value if necessary.
• the structure of Figs. 1 and 4 is further discussed in applicant's copending application entitled “Extended Range Air Fuel Ratio Sensor", filed on even date herewith.

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• Chemical & Material Sciences (AREA)
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• Physics & Mathematics (AREA)
• Chemical Kinetics & Catalysis (AREA)
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• Immunology (AREA)
• Pathology (AREA)
• Measuring Oxygen Concentration In Cells (AREA)

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• 1983
  • 1983-07-18 WO PCT/US1983/001105 patent/WO1985000660A1/en not_active Application Discontinuation
  • 1983-07-18 EP EP19830902708 patent/EP0148829A4/de not_active Withdrawn
  • 1983-07-18 JP JP50281983A patent/JPS60501871A/ja active Pending

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