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, H2, 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 02 partial pressure adjacent to the inner electrode is given by PREF- PEX' 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 (PREF/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 PEX and accor¬ dingly of exhaust gas A/F. An advantage of this device is its simplicity. A disadvantage is its low sensitivity to PEχ because of the logarithmic function. This disadvan- tage is offset near the stoichiometric A/F because PEX 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 PEX 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 02 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, Ip, through the cell as well as the voltage Vp 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 Ip 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 02 to be pumped into the partially enclosed volume from the reference atmosphere. This 02 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 02 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, Vg, versus pump cell current Ip 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 Ip 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 Ip. 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 Ip 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 Vs, is again given by Equation (1) where PREF is replaced by Pv which represents the near equilibrium oxygen partial pressure within volume v resulting from the reaction of pumped oxygen and partially reacted HC. With pv ** PEX n this case,
a - (RT/4F) In (Pv/PEχ). (2)
Fig . 2 shows a plot of induced EMF, Vs , versus pump current, Ip , at different r ich air fuel ratio values. The EMF is low for small pump currents and increases with
OMFI
<
Ip. 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 Ip 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 Ip 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^, R2 and capacitor C control the gain and frequency response of amplifier A so that A will always generate enough pump current Ip 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 Ip 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 R3, form the inputs to correction circuitry 141, to adjust Ip 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.