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

Definitions

• This invention relates to determining the composition of a gaseous atmosphere.
• the stoichiometric mixture is one in which the mass of air present contains just enough oxygen to react with the mass of hydrocarbons present so that there is the minimum amount of both oxygen and hydrocarbons remaining.
• the air fuel ratio (A/F mass of air/mass of fuel) at the stoichiometric point is approximately 14.6. If, for example, an engine was running lean of stoichiometry (A/F > 14.6), there would be an excess of air in the "charge" burned in the cylinder of an internal combustion engine and the exhaust gas would contain a substantial oxygen partial pressure. If rich operation was occurring (A/F ⁇ 14.6), the exhaust gas would contain unreacted or partially reacted hydrocarbons and very low oxygen partial pressure.
• the equilibrium oxygen partial pressure in the exhaust gas can change by a great amount (as much as 20 orders of magnitude) as one moves from lean to rich operation.
• This large change forms the basis for detecting the stoichiometric air fuel ratio with an exhaust gas oxygen sensor.
• the electrical output of such a sensor can then be fed back to an electrically controllable carburetor or fuel injection system for maintaining engine operation at the stoichiometric point.
• operation at this point frequently offers a reasonable compromise for minimizing regulated exhaust gas emissions and maximizing engine performance.
• 3,514,377 to Spacil et al relate to the measurement of oxygen (O2) concentrations using solid electrochemical devices.
• the active material in these devices may be ceramic zirconium dioxide adapted for the conduction of oxygen ions. Electrochemical cells made from this material are suitable at elevated temperatures for oxygen sensing and pumping applications.
• the mode of operation of the Heijne device can be described as an oxygen counting mode in which oxygen partial pressure is determined on a sampling basis.
• a constant current is applied to an electrochemical cell which forms part of the enclosure of a volume for a period of time to , for the purpose of electrochemically pumping out most of the oxygen from that volume.
• the ambient atmosphere is established within the volume prior to the pump out by means of a leak.
• An additional electrochemical cell which serves as a sensor of the reduced oxygen partial pressure within the volume and which also constitutes a portion of the enclosure, provides a signal indicating when oxygen has been sufficiently depleted from the volume (see Fig. 4 of Heijne). Knowing the temperature, enclosed volume, pump out current and time allows one to calculate the number of oxygen molecules within the enclosure from the ideal gas law.
• the number of oxygen molecules is in turn proportional to the desired oxygen partial pressure. If a constant pump current is used, the pump out time to is proportional to the oxygen partial pressure. If a constant current is not used, then the integral of the pump out current over the pump out time is proportional to the oxygen partial pressure.
• the Heijne device can provide an output which is linearly proportional to the oxygen partial pressure. This is superior, for example, to single oxygen concentration cells used as sensors which give an output (EMF) proportional to the natural logarithm of the oxygen partial pressure In (PO 2 ).
• a potential disadvantage of the Heijne device is response time.
• the leak connecting the ambient to the enclosed volume must be small so that during the pump out of oxygen, no significant amount of oxygen leaks into the volume to cause an error in the count of molecules (i.e., to erroneously increase to ).
• the leak may take a long time, t v , for the ambient to reestablish itself with the volume after a pump out. If the changes in the oxygen partial pressure in the ambient occur rapidly with respect to this refill time, the device would not be able to follow these changes in repetitive operation.
• Figs. 1 and 2 of the drawings illustrate a known oxygen pumping sensor in which ionically conducting zirconium dioxide material 1 with thin platinum electrodes 2 and 3 form an electrochemical cell which with additional ceramic structure 4 defines an enclosed volume 6.
• the ambient atmosphere can establish itself within the volume by means of a leak opening 5.
• a battery 7 is attached to the electrodes by means of lead wires 8 and 8'.
• a voltmeter 10 and ammeter 9 are provided to determine the voltage drop across the pump cell and the current flowing through it.
• the operation is different.
• a pump voltage V to remove oxygen from an enclosed volume 6 until the pump current saturates.
• the saturated current is proportional to oxygen partial pressure or concentration.
• the flow of oxygen through leak opening 5 equals the pump current times a proportionality constant.
• the current saturates because the leak aperture in combination with the platinum electrode 2, the cathode, will only allow a limited (saturated) amount of oxygen to enter and be electrochemically pumped from the volume per unit time.
• the saturated current value depends on the properties of the electrode 2, the device calibration may be subject to drift as these properties may change during the sintering and wear of this thin layer. For some engines it is useful to operate lean of the stoichiometric A/F ratio for the purpose of reducing fuel consumption.
• Oxygen partial pressure varies in a systematic way in the lean region and this can form the basis for determining lean A/F.
• the variation in oxygen, partial pressure in the appropriate lean A/F region is not large (in comparison to the changes occurring near stoichiometry), so that suitable oxygen sensors with sensitivities greater than the natural logarithm of oxygen partial pressure are desirable for accurate measurement in the desired A/F range.
• Oxygen partial pressure sensors for engines operating lean of stoichiometry are taught in U.S. Patent Nos. 4,272,331 and 4,272,330 to R. E. Hetrick and U.S. Patent No. 4,272,329 to R. E. Hetrick et al.
• the sensors (shown as prior art in Fig. 3 of the drawings) are placed entirely in the exhaust gas stream and include two oxygen ion conducting electrochemical cells 11 and 12, a pump cell and a sensor cell, which in part provide the enclosing structure of a nearly enclosed volume 13. A portion of the remaining structure can be a hollow ceramic tube 14.
• the cells can be attached to the end faces of the tube by ceramic glue 16.
• a small aperture 17 in the enclosing structure allows the exhaust gases, containing oxygen in a percentage to be determined, to leak into the volume.
• Lead wires 18 are affixed to electrodes 15 attached to each side of electrochemical cells 11 and 12.
• the Hetrick and Hetrick et al patents describe various external circuitry which can be coupled to the sensors to permit operation in modes including an oscillatory mode, a transient mode, and a steady-state mode. When operated in one of these modes, this device can be of great advantage in lean operation compared to the single-cell sensor since it affords a linear or greater sensitivity to oxygen concentration. Further, the various modes offer other advantageous features such as low temperature sensitivity and, in one case, independence from variations in absolute pressure.
• oxygen is electrochemically pumped into or out of the enclosed volume at a rate given by the pump-cell current, I p .
• oxygen diffuses into or out of the volume by means of the leak aperture.
• the ambient oxygen partial pressure which in turn is proportional to the A/F ratio, is dependent upon the relationship between I P and V S .
• HC partially reacted hydrocarbons
• oxygen pumping ceils one can determine the A/F by measuring the rate or amount of oxygen which must be delivered to cause a measurable reaction with the partially reacted HC.
• U.S. Patents 4,224,113 and 4,169,440 describe single-cell structures which combine electrochemical pumping of oxygen in zirconium oxide devices with the measurement of the current through, and potential difference across, that device to provide a measure of both lean and rich A/F values.
• single-cell devices may be subject to significant loss of calibration (drifting) or deterioration with extended use as would be required in automotive applications.
• the potential drop across the pump cell, which for these devices is a critical parameter in establishing A/F, can be significantly affected by the quality of the cell electrodes. This arises because more or less potential difference may be required to assure that oxygen is passed through a thick or thin electrode at the necessary rate. Such electrode polarization phenomena are common.
• this electrode contribution to the potential difference may vary with time as the electrode sinters or otherwise deteriorates under high temperature usage. Further, the ohmic contribution to the potential difference across the cell will vary exponentially with temperature requiring tight temperature control causing possible penalties in cost and performance.
• An advantage of two-cell structures such as those described by the Hetrick and Hetrick et al. patents is that the pump-cell potential difference is not a critical parameter thereby lessening the effects of electrode deterioriation and temperature.
• different sensor structures and different external circuitry are especially advantageous for A/F measurements in particular limited A/F regions.
• Known sensors which apply to a broader range may not possess the desirable features associated with the devices covering a more limited range.
• a method for generating a signal indicative of A/F of an internal combustion engine generating exhaust gases includes using selected portions of an exhaust gas oxygen sensor having a substantially enclosed volume between two electrochemical cells. A first electrochemical cell is exposed to an exhaust gas and a second electrochemical cell is exposed on one side to a reference atmosphere and on the other side to the exhaust gas.
• the method can measure an A/F ratio in an extended A/F ratio range by determining A/F ratios rich of stoichiometry, lean of stoichiometry, or at stoichiometry using selected portions of the exhaust gas oxygen sensor.
• Measuring the A/F ratio includes determining an A/F ratio rich of stoichiometry by using the second electrochemical cell as an oxygen pump, the first electrochemical cell as a voltage generator, and the reference atmosphere as a source of oxygen. Additionally, the method measures A/F ratio by determining an A/F ratio lean of stoichiometry by using the one electrochemical cell as an oxygen pump, the other electrochemical cell as a voltage generator, and the ambient adjacent the pump as a sink of oxygen. Finally, this method for determining A/F ratio includes determining A/F ratio at stoichiometry by using the second electrochemical cell exposed on one side to the exhaust gas and on the other side to a reference atmosphere as a voltage generator.
• a method in accordance with an embodiment of this invention can be used with different measurement techniques to determine exhaust gas A/F over a wide range of values including those richer than, leaner than, and near the stoichiometric air fuel value.
• the method has a "universal" air fuel sensing characteristic.
• the method allows the use of measurement techniques which are particularly advantageous in each of the three ranges.
• Figs. 1, 2 and 3 show the construction of prior art electrochemical oxygen pumping devices
• Fig. 4 is a schematic cross section of a sensor in accordance with a first embodiment of this invention.
• Fig. 5 is a graphic representation of the sensor cell voltage, V B , versus an air fuel ratio, A/F, for the sensor shown in Fig. 4;
• Fig. 6 is a schematic drawing of a sensor cell voltage, V A , versus pump cell current, I p , at various rich air fuel values for a sensor in accordance with Fig. 4;
• Fig. 7 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 rich air fuel ratios, A/F, in accordance with the sensor of Fig. 4;
• Fig. 8 is a schematic diagram of a sensor device, similar to that of Fig. 4, and external circuitry in accordance with an embodiment .of this invention for measuring rich or lean A/F ratios;
• Fig. 9 is a schematic cross section of a sensor in accordance with the second embodiment of this invention.
• Fig. 10 is a schematic diagram of a sensor device, similar to that of Fig. 4, and external circuitry in accordance with an embodiment of this invention for measuring A/F ratios near stoichiometry.
• an air fuel (A/F) sensor 110 includes an electrochemical cell 111 including a disk-like electrolyte 112 of a solid ionic conductor of oxygen such as Y 2 O 3 doped ZrO 2 .
• Cell 111 also includes two thin porous catalytic platinum electrodes 113 with attached lead wires 114.
• 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, 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 atmosphere, the exhaust gas, can establish itself within the volume v.
• Electrochemical cell 121 has a thimble-like tubular shape closed at one end thereby defining a reference volume and exposing one side of cell 121 to a reference atmosphere.
• a flat disc-shaped electrolyte 122 has a tubular structure 131 attached to it to form the thimble-like shape.
• the electrolyte itself may have a thimble-like shape.
• cell 112 and spacer 125 might be made from a single piece of material or fabricated from two separate components as shown.
• a sensor supporting structure 128 provides a seal between exhaust and reference atmospheres and structural support and protection as well as allowing for attachment to the exhaust pipe wall 127. 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 a 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.
• Air fuel sensor 140 of Fig. 9 has an electrochemical cell 141 with an electrolyte 142 and electrodes 143 attached to lead wires 144. Sensor 140 also has a second electrochemical cell 145 with an electrolyte 146 coupled to electrodes 147 which are connected to lead wires 148. A spacer 149 separates cell 141 from cell 145. An aperture 150 in cell 141 provides access from an exhaust atmosphere into the enclosed volume of sensor 140. A generally cup-shaped retaining structure 151 retains a metal metal-oxide mixture 152 adjacent to one side of electrochemical cell 145.
• Air fuel sensor 140 is positioned completely within the exhaust gas stream and can be mounted on a support structure 153 which is mounted in an exhaust pipe wall 154. Use of air fuel sensor 140 provides for fabrication simplicity and attendant reduced cost since no seal for sensor 140 is required between the exhaust and exterior atmosphere and the entire device can be contained within the exhaust gas.
• air fuel sensor 110 can be used with two different measurement techniques to determine exhaust gas air fuel ratio over a wide range of values including those richer than, leaner than and near the stoichiometric air fuel value.
• the device can be considered to have "universal" sensing characteristics.
• a steady-state oxygen-pumping mode is used for an extended range of rich and lean air fuel ratio values.
• the previously described single electrochemical cell technique is used near stoichiometry.
• the structure of the device of Fig. 4 permits use of multiple measurement techniques so that the functional advantages of each technique can be realized in a particular air fuel ratio region of application.
• air fuel sensor 110 of Fig. 4 When air fuel sensor 110 of Fig. 4 is used in connection with internal combustion engine operation at stoichiometric and near lean operation, such as air fuel ratios in the range of about 14.6 to about 17, leads 114 to cell 111 are disconnected and air fuel sensor 110 operates as a single electrochemical cell sensor previously described in connection with sensing stoichiometric air fuel ratios.
• the equilibrium oxygen partial pressure for the exhaust gas, P EX is established at the catalytic electrode 123 of cell 121 within volume v.
• P REF being equal to 0.2 atmospheres
• an EMF, V B is generated across cell 121 given by the Nernst equation:
• V B (RT/4F) In (P REF /P EX ) (1)
• Advantageous modes of operation for lean air fuel ratios greater than about 15.5 are the steady-state, oscillatory or transient operating modes described by U.S. Patents 4,272,329; 4,272,330 and 4,272,331.
• oxygen is pumped into or out of the enclosed volume v by a pump cell, e.g. cell 121, while changes in the EMF induced on the other "sensor" cell, e.g. cell 111, are monitored. Due to the change in oxygen pressure within v from the combined effects of oxygen pumping and oxygen diffusion through leak aperture 126, systematic relationships occur between the pump-cell current I P and the "sensor" cell EMF which provide a basis for oxygen sensing with high sensitivity in the lean region.
• ⁇ is proportional to the oxygen diffusion coefficient and the area of the leak aperture.
• the circuitry is of a simple servo feedback nature in which an amplifier A produces an output voltage and current which causes oxygen to be electrochemically pumped from the enclosed volume by the action of the pump cell.
• a known resistor R 3 is in series with the pump cell so that the magnitude of the pump current I P , can be determined by measuring the voltage across R 3 .
• V S (RT/4F)ln(P EX /P V ) (4)
• the essential feature allowing the use of the device as a sensor is the observation that I L is related to P V as shown in Eq. (5).
• I L ⁇ (P EX -P V ) (5)
• ⁇ is a constant characterizing the leak conductance. This relation as well as the magnitude of ⁇ can be established in the laboratory by varying I P while using calibrated gases to set P EX , and measuring V S which through Eq. (4) allows one to compute P V .
• the constant ⁇ is found to increase with T, the area of the leakage aperture, and the chemical nature of the carrier gas (e.g., N 2 or CO 2 ), and to be inversely proportional to the absolute pressure P in a manner indicating that oxygen is leaking into the volume by the gaseous diffusion mechanism. If one now solves Eq. (4) for P V and substitutes the result in Eq. (3) one finds
• I P P EX ⁇ (-1exp(-4FV S /RT)) (6)
• f(V S ,T) is a function of V S and T, so that if V S and T are held constant, I P is linearly proportional to the percentage of oxygen in the ambient.
• Heater 129 or other suitable means can be used to maintain T at a constant value while the remainder of the circuitry in Fig. 8 acts to maintain V S at a constant value by continually adjusting the value of I P as required. This is done by applying V S to the amplifier A, which in combination with suitable resistors R 1 , R 2 , and R 4 , and capacitor C produces an output proportional to the difference between V S and an adjustable reference voltage V A .
• the polarities are chosen so that the changes in the output current (I P ) act to reduce the difference between V S and V A .
• Capacitor C is chosen appropriately to damp out the effects of very sudden oxygen percentage changes and to prevent oscillations which are common in undamped servo feedback circuits. Other circuits can be devised to perform the identical control function.
• the proportionality constant between I P and ⁇ could be determined using calibrated gases. With this constant, the voltage across R 3 would serve to specify I P and hence the percentage of oxygen.
• the proportionality constant varies somewhat with the nature of the carrier gas. Accordingly, large variations in the composition of the carrier gas would have to be accounted for in accurate measurements.
• a heater 129 is used to maintain the temperature of the sensor and its adjacent gaseous surroundings within a sufficiently narrow range of values that a predetermined accuracy of the oxygen percentage measurement can be maintained with a single calibration constant appropriate for that narrow range of temperatures.
• the "heater” may need to include a more elaborate electrical heating system in which a temperature sensor in the vicinity of the device, such as a thermocouple, provides the input to an electrical temperature regulator whose output activates the heater to a variable degree sufficient to maintain the temperature sensor output (or equivalently, the temperature) equal to some constant reference value present in the regulator.
• a temperature sensor 170 may be used to form one input of temperature correction circuitry 171 whose other input is a measure of I p .
• the purpose of the circuitry is to correct I p for the changes in the device calibration constant resulting from changes in the temperature.
• the output of the circuitry can be a convenient electrical quantity, such as a voltage, whose magnitude is proportional to oxygen percentage regardless of temperature.
• the correction circuitry may need to encompass the facilities of a small computer.
• a method to determine rich air fuel ratios with air fuel sensor 110 includes causing oxygen to be pumped into v from the reference atmosphere at a rate given by I p . Simultaneously, the oxygen partial pressure within v is decreased by oxygen diffusion through leak aperature 126 and chemical reaction at interior catalytic electrodes 123 and 113 with the partially reacted HC which continuously diffuses into volume v through leak aperture 126. As pump cell current I P increases, the equilibrium oxygen partial pressure within volume v increases causing an EMF to be induced across electrochemical cell 111. The magnitude of this EMF, termed V A , is again given by Equation 1 where P REF 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. Since
• Figure 6 shows a plot of induced EMF, V A , versus pump current, I P , at different rich air fuel ratio values.
• the EMF is low for small pump currrents and increases with
• V A (REF)
• the magnitude of I P will be an increasing function of cell volume and leak aperture size. Measurement of A/F and subsequent feedback control of engine A/F could be achieved in a manner analogous to that employed for lean operation.
• the circuitry of Fig. 8 is applicable with appropriate changes in the sign and magnitude of V A as required by various device parameters.
• the measured voltage across resistor R 3 provides a measure of I P which in combination with the calibration curves of Fig. 6 determine A/F.
• standard electronic techniques can be used to accomplish actual A/F feedback control, temperature correction, etc.
• V A V A
• the measured and desired values of pump current are compared and a feedback or error signal, sent to an electrically controlled carburetor or fuel injection system, accomplishes feedback control.
• the most appropriate technique in this region utilizes the conventional single electrochemical cell approach with a reference electrode at atmospheric oxygen partial pressure. Feedback control is achieved by comparing the output of the cell with that voltage corresponding to the desired air fuel ratio which is a known value and can be made electronically available in computer memory.
• a single unit, sensor 110 provides high sensitivity to air fuel ratio both over an extended range of lean and rich conditions using a pumping mode of operation and near stoichiometry using a single electrochemical cell.
• cell 111 as the pump, removing oxygen from v and returning it to the exhaust, and cell 121 as the "sensor" in lean operation. This is possible with only a small modification to the operating results. As an example, one finds in the steady state mode that
• I P ⁇ (P EX - P REF e-(V s /V o )) (8)
• V s V REF is assumed to be constant
• I P is still proportional to P EX although offset by a constant amount from the value found in Equation (2).
• a judicious choice of V s will still allow convenient lean operation with high sensitivity.
• the advantage of this reversal of pump and sensor cells would be to eliminate current flow in cell 121 which may also be used as the sensor cell in subsequent stoichiometric operation. It is known that if current flow is too large in oxygen ion conductors, electrolyte or electrode deterioration can occur. This in turn could cause false or spurious EMF's to develop under open circuit conditions so that subsequent operation as a sensor cell would be compromised. In this case, however, the fact that the sensor cell electrode is not immersed in the exhaust results in the air fuel ratio calibration curve which has some small sensitivity to absolute exhaust pressure. In the embodiment shown in Fig. 9, the air reference is replaced by an alternate reference having metal-metal oxide mixtures 152 (e.g.
• the two-cell structure is similar to that shown in Fig. 4 except that the metal-metal oxide mixture is retained adjacent to the cell 145 reference electrode 147 by a retaining structure 151.
• P M/MO REF
• A/F sensing near stoichiometry is accomplished by disconnecting the lead wires 114 of cell 111 while attaching the lead wires 124 of cell 122 to appropriate voltage measuring and signal processing circuitry indicated schematically by 161.

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• 1983
  • 1983-07-18 WO PCT/US1983/001098 patent/WO1985000659A1/fr not_active Application Discontinuation
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  • 1983-07-18 JP JP50307183A patent/JPS60501872A/ja active Pending

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