STEADY-STATE METHOD FOR DETERMINING RICH AIR/FUEL RATIOS

Abstract: A steady-state method for determining air to fuel ratios rich of stoichiometry includes generation of a signal proportional to the concentrations of unreacted and partially reacted hydrocarbons. Two electrochemical cells (111, 121) are spaced from one another and define 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 two electrochemical cells is exposed to the volume. A second side of one of the electrochemical cells is exposed to the exhaust gases. A second side of the other of the electrochemical cells is exposed to a reference atmosphere usually air. External circuitry attached to the electrodes of both cells causes one electrochemical cell to pump oxygen into the partially enclosed volume from the reference atmosphere. This oxygen reacts with unreacted and partially reacted hydrocarbons within the volume which in turn causes an EMF to be generated across the other electrochemical cell. This EMF functions as the input to the external circuitry. The circuitry pumps oxygen into the volume at rate which will keep the induced EMF fixed at an arbitrary value. The magnitude of the steady-state current required to accomplish this task is proportional to the concentrations of unreacted and partially reacted hydrocarbons in the exhaust gas and hence is inversely proportional to rich A/F ratio.

Description: STEADY-STATE METHOD FOR DETERMINING RICH AIR/FUEL RATIOS Technical Field This invention relates to determining the composition of a gaseous atmosphere. Background Art 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. For an automotive gasoline engine 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. This relationship is the basis for the use of high temperature oxygen sensors to determine A/F at and lean of stoichiometry. For rich operation (A/F < 14.6) the equilibrium partial pressure of oxygen is very small and the exhaust gas contains a substantial partial pressure of unreacted hydrocarbons and partially reacted hydrocarbons such as hydrogen, H 2 , and carbon monoxide, CO. At thermodynamic equilibrium, the concentrations of these species increase monotonically with decreasing A/F and thereby provide a measure of rich A/F- High temperature oxygen sensors using electro¬ chemical cells fabricated from the ceramic solid electro¬ lyte zirconium dioxide, Zrθ2, doped with Ϊ2^3 (° r other comparable materials) is well known. For example, U.S. 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. In this device, 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' tlιe °2 Partial pressure in the exhaust gas, is adjacent to the outer electrode. The EMF (=V) developed across the cell in this configuration is given by the Nernst equation: V * ( RT/4F ) In (P R EF/PEX> <*> where R is the universal gas constant, T is the absolute temperature, and F is the Faraday constant. Thus, 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 χ 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. Thus a substantial variation of V ( ^ 1.0 volt) characterizes this specific A/F ratio in automotive applications. Away from the stoichiometric A/F, the variation of P EX with A/F is much weaker and the single cell device is less sensitive to changes in A/F. OM One method of enhancing sensitivity is through the use of O2 pumping devices also employing Zrθ2 electrochemical cells. Thus, U.S. Patents 3,907,657 to Heijne; 3,514,377 to Spacil et al; 4,272,329 to Hetrick et al describe one, two or multiple cell structures which combine oxygen pumping with one cell and EMF measurements with other cells to effect measurements of O2 concentra¬ tion at higher sensitivity. In the case of the Hetrick et al patent, the structure is especially suited for measur- ing lean A/F in automotive applications. In gaseous environments rich of stoichiometry, oxygen pumping devices can also be used to measure A/F with higher sensitivity. In this case, one must use a structure where one measures the rate at which 0 2 must be delivered to effect a measurable reaction with the unreacted or partially reacted HC which are present in large amounts. 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. When O2 is pumped at a high enough rate from an O2 "reservoir" side of the cell to the "reaction" side of the cell to bring the O2 and HC concentrations on the "reaction" side of the cell close to the stoichiometric ratio, then a significant variation (on the order of 1 volt) will occur in Vp signalling the passage through stoichiometry. More current will be required to achieve this condition for lower values of A/F. In this way, the I p value required to achieve the voltage variation provides a measure of rich A/F- OM However, such single cell devices may be subject to significant loss of calibration (drifting) or deterior¬ ation with extended use as would be required in automotive applications. The voltage 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 occurs because more or less voltage may be required to assure that 0 is passed through a thick or thin electrode at the necessary rate. Such electrode polarization phenomena are common. Thus, this electrode contribution to the voltage may vary with time as the electrode sinters or otherwise deteriorates under high temperature usage. Further, the ohmic contribution to the voltage across the cell will vary exponentially with temperature requiring tight temperature control with probable penalties in cost and performance. On the other hand, with two cell structures, the voltage drop across the pumping cell is frequently of little importance thus lessening the effects of electrode deterioration and temperature. As a result, a two cell structure with a corresponding appropriate measurement technique could be especially advantageous for high sensitivity rich A/F measurements. These are some of the problems which this invention overcomes. DISCLOSURE OF THE INVENTION In accordance with an embodiment of this invention 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. OMPI 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. In accordance with the method, 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. BRIEF DESCRIPTION OF THE DRAWINGS 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; OM 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; and 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. BEST MODE FOR CARRYING OUT THE INVENTION Referring to Fig. 1, 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 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, 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 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. As pump cell current I p increases, the steady-state oxygen partial pressure within volume v increases causing an EMF to be induced across electro¬ chemical cell 111 (sensor cell). 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. With p v ** P EX n th is case, a - (RT/4F) In (P v /P E χ). (2) 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 OMFI < I p . For lower air fuel ratios 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 the I p required to cause the EMF on electro- chemical cell 111 to reach an arbitrary reference value V(REF) (maintained in external circuitry) will increase systematically with decreasing (i.e. richer) air fuel ratio as indicated in Fig. 3. Such a calibration curve provides the basis for measuring rich air fuel ratios. The choice of V(REF) would be influenced by a number of design considerations principally involving response time. Note also that the required I p will be an increasing func¬ tion of cell volume and leak aperture size. A convenient circuitry for implementing this method with the structure of Fig. 1 is shown in Fig. 4. In Fig. 4 the supporting structure is not shown for clarity. 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. If the current is too high or low, intake fuel could be increased or decreased, respectively, thereby accomplish¬ ing feedback control. Also, shown is 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. Various modifications and variations will no doubt occur to those skilled in the various arts to which this invention pertains. For example, the electrochemical cell shape 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 making a measurement of air to fuel ratio rich of the stoichiometric value in an ambient environment containing unreacted and partially reacted hydrocarbons including the steps of: establising an enclosed volume with restricted access to the ambient environment by means of supporting a first electrochemical cell and a second electrochemical cell, at spaced positions and defining therebetween a volume, providing communication from the ambient environment to the volume through an opening between the volume and the ambient; establishing a reference oxygen partial pressure; exposing a first side of said first electrochemical cell to the volume; and exposing a first side of said second electrochemical cell to the volume and a second side to a reference oxygen partial pressure. 2. A method as recited in claim 1 further comprising: exposing a second side of said first electrochemical cell to the ambient; and passing a pump current through said 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 said first electrochemical cell thereby causing an EMF to be generated across said first electrochemical cell. OMPI 3. A method as recited in claim 2 wherein said step of passing a current includes: providing just enough pump current to said second electrochemical cell so that the EMF induced across said first electrochemical cell is maintained at a constant reference voltage value. 4. A method as recited in claim 3 further comprising: measuring the magnitude of the pump current, the pump current being proportional to the concentrations of unreacted and partially reacted hydrocarbons in the ambient and also being inversely proportional to a rich A/F ratio. 5. A method as recited in claim 4 further comprising the step of: maintaining the temperature of the enclosed volume and adjacent region so that a single calibration constant is appropriate for the range of temperatures maintained. 6. A method as recited in claim 5 further comprising the step of: measuring the temperature in the region of the electrochemical cells and correcting the measurement of the A/F ratio for the dependence of the oxygen pumping current on the temperature. G-.-TPI 7. A method of making a measurement of air to fuel (A/F) ratio rich of the stoichiometric value in an ambient environment containing unreacted and partially reacted hydrocarbons including the steps of: establishing an enclosed volume with restricted access to the ambient environment by means of supporting a first electrochemical cell and a second electrochemical cell at spaced positions and defining therebetween a volume, each of said first and second electrochemical cells having opposing electrodes; providing communication from the ambient environment to the volume through an opening between the volume and the ambient; exposing a first side of said first electrochemical cell and a first side of said second electrochemical cell to the volume; establishing a reference oxygen partial pressure adjacent to a second side of said second electrochemical cell; exposing a second side of said second electrochemical cell to the ambient environment; passing a pump current through said 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 said first electrochemical cell thereby causing an EMF to be generated between the opposing electrodes of said first electrochemical cell; providing an amount of pump current to said second electrochemical cell so that the EMF induced across said first electrochemical cell is maintained at a constant reference voltage value; OMPI establishing and maintaining the reference voltage value in an external circuit coupled to said first electrochemical cell; measuring the magnitude of the pump current; and determining the rich A/F ratio from the magnitude of the pump current. 8. A method as recited in claim 7 further comprising the steps of: maintaining the temperature of the enclosed volume within a predetermined range of temperatures; selecting a calibration constant as a function of the maintained temperature to be used when determining the rich A/F ratio; measuring the temperature in the region of the electrochemical cells; and correcting the determination of the A/F ratio for dependence of the pumping current magnitude on the temperature. 9. A method as recited in claim 7 wherein the step of establishing a reference oxygen partial pressure includes exposing the second side of said second electrochemical cell to the atmosphere.

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