MEASURING AN EXTENDED RANGE OF AIR FUEL RATIO

Abstract: A method of using selected portions of an exhaust gas sensor (110) having a substantially enclosed volume, v, between two electrochemical cells (111, 121). One electrochemical cell (111) is exposed to an exhaust gas and another electrochemical cell (121) is exposed to a reference atmosphere. An air/fuel ratio rich of stoichiometry uses electrochemical cell (121) as an oxygen pump, electrochemical cell (111) as a voltage generator, and the reference atmosphere as a source of oxygen. An air/fuel ratio lean of stoichiometry uses electrochemical cell (121, or 111) as an oxygen pump, electrochemical cell (111, or 121) as a voltage generator, and the ambient adjacent the oxygen pump as a sink oxygen. An air/fuel ratio at stoichiometry uses electrochemical cell (121) as a voltage generator.

Description: MEASURING AN EXTENDED RANGE OF AIR FUEL RATIO Technical Field This invention relates to determining the composition of a gaseous atmosphere. Background Art It is known to use high temperature oxygen sensors in the determination of a stoichio etric air fuel mixture in the exhaust gases of automobile internal combus¬ tion engines. 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 stoichio¬ metry (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) r the exhaust gas would contain unreacted or partially reacted hydrocarbons and very low oxygen partial pressure. In particular, 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. Depending on engine type, operation at this point frequently offers a reasonable compromise for minimizing regulated exhaust gas emissions and maximizing engine performance. There are known high temperature oxygen sensors utilizing a single oxygen electrochemical concentration cell (usually made from zirconium oxide) and requiring the use of a reference atmosphere (usually air) which are suit¬ able for determining the stoichiometric air fuel ratio in a high temperature automotive environment. These devices give an output (EMF) proportional to the natural logarithm of the oxygen partial pressure. Despite their low sensi- tivity to oxygen partial pressure, the large change in oxygen partial pressure at the stoichiometric point allows their useful implementation. U.S. Patents 3,948,081; 3,738,341; 4,112,893; 4,210,509; and 4,107,019 relate to oxygen sensors of this type. U.S. Patent . Nos. 3,907,657 to Heijne and 3,514,377 to Spacil et al relate to the measurement of oxygen (O2) concentrations using solid electrochemical devices. For applications at elevated temperatures ( > 500° C), for example,, as might be encountered in the exhaust gases of furnaces or automobiles, the active material in these devices may be ceramic zirconium dioxide adapted for the conduction of oxygen ions. Electro¬ chemical 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 OMPI constant current is applied to an electrochemical cell which forms part of the enclosure of a volume for a period of time tp , 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 electro¬ chemical 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 tempera¬ ture, 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 t p 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 concentra¬ tion cells used as sensors which give an output (EMF) proportional to the natural logarithm of the oxygen partial pressure In (PQ )• A potential disadvantage of the Heijne device is response time. For this measurement procedure, the leak connecting the ambient to the enclosed volume must be small so that during the pump out of oxygen, no signifi¬ cant amount of oxygen leaks into the volume to cause an error in the count of molecules (i.e., to erroneously increase tp ). However, if the leak is made small, it 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. In the case of the teachings of U.S. Patent No. 3,698,384 to Jones, the purpose is to measure oxygen partial pressure in a feedgas. This is done by measuring an electrochemical cell pumping current while holding the sensor cell voltage a constant. However, the flow rate of the feedgas must be kept constant. If the flow rate should attempt to vary, there is a relatively elaborate flow control circuit to keep the flow rate a constant. This scheme, which also employes a reference atmosphere, is relatively unsuitable for application in an auto exhaust where the exhaust flow rate would change substantially with RPM. Figs. 1 and 2 of the drawings illustrate a known oxygen pumping sensor in which ionically conducting zir¬ conium 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 volt¬ meter 10 and ammeter 9 are provided to determine the voltage drop across the pump cell and the current flowing through it. Although similar to structure to Fig. 5 of U.S. Patent No. 3,907,657, the operation is different. Here one applies 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. This is a steady-state device. When steady state is reached, the flow of oxygen through leak opening 5 equals the pump current times a proportionality constant. The current saturates because the leak aperture in combina¬ tion 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. To the extent that 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. However, 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 oscil¬ latory 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. In these modes, oxygen is electrochemically pumped into or out of the enclosed volume at a rate given by the pump-cell current, I p . Simultaneously, oxygen diffuses into or out of the volume by means of the leak aperture. The oxygen fluxes due to leakage and pumping alter the oxygen partial pressure within the volume relative to the ambient so that an EMF (= V s ) develops across the sensor cell. 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 . Further, in addition to the previously discussed stoichiometric and lean air/fuel operation, there are occasions where engine operation rich of stoichiometry is desired. In this region, the amount of partially reacted hydrocarbons (HC) such as carbon monoxide and hydrogen which increase with decreasing A/F can serve as a measure of A/F. Using 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. Thus, 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. However, such 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. Thus, 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. ,O Thus it can be appreciated that 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. In any case, it would be desirable to have an exhaust gas oxygen sensor which could indicate engine A/F over an extended range of rich and lean A/F values, including stoichiometry, which incorporates the most useful properties of different sensors designed for limited ranges. These are some of the problems this invention overcomes. DISCLOSURE OF THE INVENTION In accordance with an embodiment of this inven- tion, 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. Addition¬ ally, the method measures A/F ratio by determining an A/F ratio lean of stoichiometry by using the one electro¬ chemical 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. As a result, 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. Hence, the method has a "universal" air fuel sensing characteristic. Further, the method allows the use of measurement techniques which are particularly advantageous in each of the three ranges. BRIEF DESCRIPTION OF THE DRAWINGS 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; and 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. BEST MODE FOR CARRYING OUT THE INVENTION Referring to Fig. 4, 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 2O3 doped Zrθ2- 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, 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. In particular, a flat disc-shaped electrolyte 122 has a tubular structure 131 attached to it to form the thimble-like shape. As a result, one side of the sensor is exposed to the exhaust gas and one side is exposed to the reference atmosphere. Alternatively, the electrolyte itself may have a thimble-like shape. In a O Π similar way, 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. Referring to Fig. 9, another embodiment in accor¬ dance with this invention replaces the air reference on one side of cell 121 of Fig. 4 with a metal, metal-oxide mixture. Air fuel sensor 140 of Fig. 9 has an electro¬ chemical 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 posi¬ tioned 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 pro- vides 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. Referring to the operation of the device of Fig. 4, 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. Hence, the device can be considered to have "universal" sensing characteristics. First, a steady-state oxygen-pumping mode is used for an extended range of rich and lean air fuel ratio values. Second, 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 applica¬ tion. The use of the single electrochemical cell for measuring A/F near stoichiometry is well known and is taught in U.S. Patent 3,948,081 to Wessel et al. The use of pumping techniques for lean A/F sensing using a two cell structure are taught by U.S. Patent 4,272,329 to Hetrick et al and U.S. Patents 4,272,330 and 4,272,331 to Hetrick. The disclosures of these patents are hereby incorporated by reference. The use of two-cell pumping techniques for rich A/F sensing are further discussed in a copending application filed on even date herewith entitled "Steady-State Method of Determining Rich Air/Fuel Ratios" by Hetrick et al. 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 voluae v. In combination with the oxygen partial pressure in the air reference, PREF' 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 RE F PEX> d> where R is the gas constant, T is the absolute temperature and F is the Faraday constant. As P EX and the corres¬ ponding air fuel ratio decrease, the cell EMF increases as shown in Fig. 5. The strong variation of exhaust gas oxygen and, correspondingly, the cell EMF in this region makes this a relatively simple and desirable technique for the stoichiometric and near lean regions of air fuel ratio operation. At larger or smaller air fuel values, however, the variation of the cell EMF with air fuel becomes too small for a desirably effective sensor operation. 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. In these modes 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 relation¬ ships 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. In particular, in the steady-state mode of operation, external circuitry causes a current I p to be passed through the pump cell 121 to withdraw just enough oxygen from v so that a constant sensor cell EMF, termed V s , is established. As the percentage of oxygen increases, so does the required pump current thereby providing a measure of the oxygen percentage and corres¬ ponding A/F. In particular, one finds that !p - σ P E X (1 " e -V s /V 0 ) (2) where V 0 - RT/4F and σ is a constant of proportionality defining the rate at which oxygen can diffuse into v through the leak aperture. For example, σ is proportional to the oxygen diffusion coefficient and the area of the leak aperture. Thus, by always passing just enough I p to keep V s a constant, I p ^ PEX^ A/F thereby allowing for high sensitivity A/F sensing. For automotive applications it is also advantageous that this technique has weak temperature and absolute pressure sensitivities. The cited patents describe device operation for both sensor and pump cells completely immersed in the exhaust gas. Sensor 110 has an analogous mode of opera¬ tion even though the exterior electrode 123 of pump electrochemical cell 121 is exposed to a reference atmosphere with high oxygen concentration as shown in Fig. 4. The reason is that the effect of the reference atmosphere is to add a small increment to the total potential difference across pump cell 121. However, only the current, I p , through the pump cell and not the potential drop across the cell is important for device operation. Accordingly, all lean operating modes described in these patents can be accomplished with the present structure where electrochemical cell 121 is used as the pump cell and electrochemical cell 111 is used as the sensor cell. Electrical operation for sensing lean A/F ratios can be further discussed with reference to Fig. 8 which shows the device wired to an external circuit. 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 R3 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 R3. After an initial transient period, a steady state is reached where the number of oxygen molecules removed by pumping is equalled by a flux (I L ) of oxygen molecules diffusing into the volume by means of the leak aperture. The oxygen is diffusing through whatever gaseous species, the carrier gas, comprises the remainder of the ambient atmosphere. This equality is expressed by Eq. (3) I L (02molecules/sec)»(I p /4e) (0 2 molecules/sec) (3) where e is the electronic charge and 4e converts I p in amps to an equivalent number of oxygen molecules/sec. At steady state P v , the oxygen partial pressure which is assumed to be constant throughout the enclosed volume, adopts a value less than P-gχ, the oxygen partial pressure in the surrounding ambient. As a result of this partial pressure difference, an electromotive force, labelled V S / will develop across the sensor cell of a magnitude given by the familiar Nernst equation as shown in Eq. (4) V s =(RT/4F)ln(P EX /P v ) (4) where R, T and F were defined previously. 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 « σ (PEX-PV) ( 5 ) where σ is a constant characterizing the leak conduc¬ tance. 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 σ 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 C0 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 r p- p EX σ d-exp(-4FV s /RT)) (6) If the gases involved obey the ideal gas law, which is an excellent approximation' at the elevated temperatures of interest, then ?EX aaP where a is the fractional number of molecules in the ambient atmosphere that are oxygen. Accordingly, if β » 100% x a then 8 is the percentage of oxygen in the ambient. Further since σ ^ (1/P) one has the result that Ip M3 f(V s ,T), (7) where f(V s ,T) is a function of V s and T, so that if Vg 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 adjust¬ ing the value of I p as required. This is done by applying Vg to the amplifier A, which in combination with suitable resistors R^, R 2 , and R4, and capacitor C produces an output proportional to the difference between Vg 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 Vg and V A . Capacitor C is chosen appropriately to damp out the effects of very sudden oxygen percentage changes and to prevent oscilla¬ tions which are common in undamped servo feedback circuits. Other circuits can be devised to perform the identical control function. In use, the proportionality constant between I p and 8 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. An advantageous aspect of this mode of oxygen measurement is the weak temperature dependence of the sensed parameter, I p . This arises because σ in Eq. (6) is an increasing function of T while the right hand factor of σ is a decreasing function of T. For a given applica¬ tion, a near cancellation of the temperature dependence over an approximate 100°C. temperature range can be achieved by a judicious choice of V s . This feature relaxes the performance requirements of any heater assembly which would be necessary in conjunction with the device to maintain the temperature constant within an acceptable range. To facilitate accuracy, it may be advantageous to account for the effects of changes in the temperature of the ambient atmosphere. This can be done in two ways. Firstly, referring to Fig. 8, 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. As a given application requires, 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 suffi¬ cient to maintain the temperature sensor output (or equivalently, the temperature) equal to some constant reference value present in the regulator. Alternatively 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 propor¬ tional to oxygen percentage regardless of temperature. Dependening on the application, the correction circuitry may need to encompass the facilities of a small computer. During operation with air fuel ratios rich of stoichiometry (A/F < 14.7), the concentrations of partially reacted HC increase with decreasing air fuel ratios thereby providing a measure of the air fuel ratio. In a manner analogous to that used for lean operation, 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 OMPI 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 R EF i- 3 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 p v > P EX in this case, the sign of the EMF will be opposite that induced by pumping action during lean air fuel ratio measurement. 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 Ip. For lower air fuel ratio an ever increasing amount of oxygen must be pumped into volume v to accomplish a significant reaction with the HC. In particular, the value of I p required to cause the EMF on electrochemical cell 111 to reach an arbitrary reference value V A (REF) (maintained in the external circuitry) will increase systematically with decreasing (i.e. richer) air fuel ratio as indicated in Fig. 7. Such a calibration curve provides the basis for measuring rich air fuel ratios. The choice of V A (REF) would be influenced by a number of design considerations, but could for simplicity be chosen equal to, but opposite in sign, to the reference voltage used in the steady-state mode for lean sensing operation. The magnitude of I p will be an increasing function of cell volume and leak aperture size. OMPI ^ b/ V -. IPO --Λ 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 R3 provides a measure of I p which in combination with the calibration curves of Fig. 6 determine A/F. As for the lean A/F measurement, standard electronic techniques can be used to accomplish actual A/F feedback control, temperature correction, etc. In summary, operation rich or lean of stoichiometry would be accomplished by pumping oxygen into or out of the enclosed volume until a predetermined V A (REF) appropriate for rich or lean conditions is achieved. With rich and lean calibration curves electronically avail¬ able as in an onboard computer, 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. Because of the highly exothermic nature of the HC-oxygen reaction, very small amounts of pumped oxygen can cause wide variations in V A at or near stoichiometry. Accordingly, 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. As a result, 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. .Alternatively, it may be advantageous to use 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 J p » σ ( p EX " p REF e-(V s /V 0 )) (8) Thus, by adjusting I p to keep V s always fixed (PREF is assumed to be constant) at an arbitrary value, 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 stoichio¬ metric 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. Ni/Niθ2, Cu/Cuθ2)- 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. This embodiment is appropriate for lean and stoichiometric operation where cells 145 and 141 act as sensor and pump cells, respectively. Since the effective oxygen partial pressure at a typical metal-metal oxide reference electrode, PJJ/MO (KEF) / s much less than P EX under lean conditions, a substantial EMF (e.g. 200-500 mV) will appear across the sensor cell 145 at » 0. .As oxygen is pumped from volume v 9 by pump cell 141, this EMF will be reduced. Choosing an appropriate EMF in this reduced range as a reference value, analysis analogous to that used in U.S. Patent 4,272,329 shows that the pump current required to keep the reference voltage constant is proportional to the percentage of oxygen in the exhaust gas thereby serving as a sensor of lean air fuel ratio as in the previously discussed cases. For near stoichio¬ metric operation the pump cell 141 is disconnected and the open circuit EMF of sensor cell 145 is monitored. As for other single-cell sensors, passage of the exhaust gas through stoichiometry is attended by a large variation in the cell EMF which is adequate to determine air fuel ratios in a narrow range. A pumping mode for rich air fuel ratio detection requires that sufficient oxygen be available. Referring to Fig. 10, A/F sensing near stoichio¬ metry 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. Figure 5 shows the EMF (=V B ) generated by cell 122 for A/F values near stoichiometry and which must be measured by the circuitry of 161. Additional electronic circuitry can be incor¬ porated in 161 to process the input EMF signal for activating a fuel injection or carburation system for feedback control of A/F. Although the measurement of A/F for rich, lean and stoichiometry are shown as separate embodiments in Figs. 8 and 10, respectively, it should be appreciated that, if desired, all the circuitry can be coupled to a single sensor 110. If such coupling is done, it may be desirable to use switches for selectively connecting and disconnecting the appropriate circuitry. Various modifications and variations will no doubt occur to those skilled in the various arts to which this invention pertains. For example, the particular electrical components to carry out the method may vary from that disclosed herein. These and all other variations which basically rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention.

Claims: We claim: 1. A method of using selected portions of an exhaust gas oxygen sensor having a substantially enclosed volume between two electrochemical cells, a first elec¬ trochemical cell exposed to an exhaust gas and a second electrochemical cell with one side exposed to a reference atmosphere and a second side exposed to an exhaust gas, said method measuring an air/fuel ratio within an extended air/fuel ratio range including the steps of: determining an air/fuel ratio rich of stoichiometry by using the second electrochemical cell adjacent the reference atmosphere as an oxygen pump, the first electrochemical cell as a voltage generator, and the reference atmosphere as a source of oxygen; determining an air/fuel ratio lean of stoichiometry by using one electrochemical cell as an oxygen pump, the ambient on the opposite side of the oxygen pump from the enclosed volume acting as a sink of oxygen, and the other electrochemical cell as a voltage generator; and determining an air/fuel ratio at stoichiometry by using the second electrochemical cell with one surface exposed to the exhaust gas and a second exposed to the reference atmosphere as a voltage generator. 2. A method as recited in Claim 1 wherein the step of determining an air/fuel ratio rich of stoichiometry includes the steps of: passing a pump current through the second electrochemical cell so that oxygen is pumped from the reference atmosphere into the enclosed volume and so that, by chemical reaction of the oxygen with the unreacted and partially reacted hydrocarbons within the volume, a difference in oxygen partial pressure is established at the first electrochemical cell thereby causing an EMF to be generated between the opposing electrodes of the first electrochemical cell; providing an amount of pump current to the second electrochemical cell so that the EMF induced across the first electrochemical cell is maintained at a constant reference voltage value; establishing and maintaining the reference voltage value in an external circuit coupled to the first electrochemical cell; measuring the magnitude of the pump current; and determining the rich A/F ratio from the magnitude of the pump current. 3. A method as recited in Claim 1, wherein the step of determining an air/fuel ratio lean of stoichiometry includes the steps of: passing a pump current through the second electrochemical cell so that oxygen is pumped from the enclosed volume into the reference atmosphere and so that, a lower oxygen partial pressure is established in the enclosed volume thereby causing an EMF to be generated between opposing electrodes of the first electrochemical cell; providing an amount of pump current to the second electrochemical cell so that the EMF induced across the first electrochemical cell is maintained at a constant reference voltage value; establishing and maintaining the reference voltage value in an external circuit coupled to the first electrochemical cell; measuring the magnitude of the pump current; and determining the lean A/F ratio from the magnitude of the pump current. OMPI IPO 4. A method as recited in Claim 1, wherein the step of determining an air/fuel ratio lean of stoichiometry includes the steps of: passing a pump current through the first electrochemical cell so that oxygen is pumped from the enclosed volume into the exhaust gas and so that, a lower oxygen partial pressure is established in the enclosed volume thereby causing an EMF to be generated between opposing electrodes of the second electrochemical cell; providing an amount of pump current to the first electrochemical cell so that the EMF induced across the second electrochemical cell is maintained at a constant reference voltage value; establishing and maintaining the reference voltage value in an external circuit coupled to the second electrochemical cell; measuring the magnitude of the pump current; and determining the lean A/F ratio from the magnitude of the pump current. 5. A method as recited in Claim 1 wherein the step of determining an air/fuel ratio at stoichiometry precludes the step of: measuring the open circuit EMF of the second electrochemical cell to determine the occurrence of a stoichiometric air/fuel ratio. 6. A method as recited in Claim 3 further comprising the step of: establishing an ambient atmosphere as the reference atmosphere. 7. A method as recited in Claim 3 further comprising the step of: establishing a metal-metal oxide material as the reference atmosphere. 8. A method as recited in Claim 2 further comprising the step of: maintaining the temperature of the enclosed volume and adjacent rgion so that a single calibration constant is appropriate for the range of temperatures maintained. 9. A method as recited in Claim 3 further comprising the step of measuring the temperature in the region of the sensor cell and correcting the measurement of A/F for its dependence on the temperature. 10. A method of using selected portions of an exhaust gas oxygen sensor having a substantially enclosed volume between two electrochemical cells, a first electro¬ chemical cell exposed to an exhaust gas and a second electrochemical cell with one side exposed to a reference atmosphere and a second side exposed to an exhaust gas, said method measuring an air/fuel ratio within an extended air/fuel ratio range including the steps of: determining an air/fuel ratio rich of stoichiometry by: passing a pump current through the second electrochemical cell so that oxygen is pumped from the reference atmosphere into the enclosed volume and so that, by chemical reaction of the oxygen with the unreacted and partially reacted hydrocarbons within the volume, a OMPI difference in oxygen partial pressure is established at the first electrochemical cell thereby causing an EMF to be generated between the opposing electrodes of the first electrochemical cell; providing an amount of pump current to the second electrochemical cell so that the EMF induced across the first electrochemical cell is maintained at a constant reference voltage value; establishing and maintaining the reference voltage value in an external circuit coupled to the first electrochemical cell; measuring the magnitude of the pump current; and determining the rich A/F ratio from the magnitude of the pump current; determing an A/F ratio lean of stoichiometry by: passing a pump current through the first electrochemical cell so that oxygen is pumped from the enclosed volume into the exhaust gas and so that, a lower oxygen partial pressure is established in the enclosed volume thereby causing an EMF to be generated between opposing electrodes of the second electrochemical cell; providing an amount of pump current to the first electrochemical cell so that the EMF induced across the second electrochemical cell is maintained at a constant reference voltage value; establishing and maintaining the reference voltage value in an external circuit coupled to the second electrochemical cell; measuring the magnitude of the pump current; determining the lean A/F ratio from the magnitude of the pump current; and _OMPI deterraining an A/F ratio at stoichiometry by using the second electrochemical cell with one surface exposed to the exhaust gas and a second surface exposed to the reference atmosphere as a voltage generator and measuring the open circuit EMF of the second electro¬ chemical cell to determine the occurrence of a stoichio¬ metric air/fuel ratio. ifE tf- OMI

Inventors: HETRICK ROBERT E, LOGOTHETIS ELEFTHERIOS M, HETRICK, ROBERT, E., LOGOTHETIS, ELEFTHERIOS, M.
Applicants: FORD MOTOR CO, FORD WERKE AG, FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1983
Main classification: G01N27
IPC codes: G01N27