A sensor for determining the stoichiometric air to fuel ratio (A/F) at the input to the cylinders of an internal combustion engine which is based on measuring the change in the workfunction of a material which occurs when the oxidizing and reducing species in the adjacent gas phase are at or near their stoichiometric ratio. In one method the sensor includes a material which is capable of thermionically emitting alkali metal or other appropriate ions into an exhaust gas atmosphere where they are subsequently collected by a nearby collector electrode and the magnitude of the emission current is measured. The interaction of the emitting surface with the gas phase reversibly changes the workfunction of the emitting surface from large to smaller values as that gas phase is varied through the stoichiometric ratio with respect to the amounts of the oxidizing and reducing species in the gas. Such a change in workfunction of the surface is accompanied by a change in the rate of the thermionic emission at the stoichiometric ratio thereby sensing that ratio in the gas phase which is proportional to the A/F at the input to the cylinders.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electrical means to measure the stoichiometric 
ratio of the concentrations of oxygen and other oxidizing gaseous species 
to the concentrations of various reducing gaseous species such as 
hydrocarbons, hydrogen and carbon monoxide as might be found in the 
automotive exhaust. 
2. Prior Art 
High temperature, solid-state, air-to-fuel ratio (A/F) sensors have seen 
widespread use in the automotive exhaust as the feedback control element 
used to maintain the A/F at the input to the cylinders of an internal 
combustion engine near the stoichiometric value so that any oxidizing and 
reducing species in the exhaust gas can most efficiently be reduced to low 
concentrations by an exhaust gas catalyst. The sensors actually determine 
the ratio of the concentrations of oxidizing and reducing species in the 
exhaust gas. This in turn is proportional, with a different 
proportionality constant for each type of fuel, to the A/F which is 
defined as the ratio of the mass of air to the mass of fuel that is 
introduced to the cylinders. 
Current sensors can be conveniently divided into those which have a 
step-like transfer function at the stoichiometric A/F ratio (such 
"stoichiometry sensors" have an output which switches from low to high 
values as the A/F passes from oxidizing (lean) to reducing (rich) 
conditions at stoichiometry) and those which have a more nearly linear 
response over a wide range of A/F on both the rich and lean side of 
stoichiometry. Because the first type of device has a nonlinear transfer 
characteristic, it is commonly used in an oscillatory or limit-cycle 
feedback control strategy. The linear transfer characteristic of the 
latter type of device makes it advantageous for use in the proportional 
feedback control of A/F. 
Most existing A/F sensors use either an electrochemical or a resistive 
principle. Thus a publication by H. Dietz, W. Haecher and H. Jahnke, in 
Advances in Electrochemistry and Electrochemical Engineering, Vol. 10, 
Wiley, N.Y., pg. 1 (1977), describes a solid state electrochemical cell 
composed, for example, of zirconium dioxide doped with yttrium dioxide, 
using platinum electrodes, shaped as a cylinder closed at one end with the 
exterior electrode exposed to the gas of interest while the interior 
electrode is exposed to a reference atmosphere of fixed oxygen 
concentration (typically air). In an automotive application this type of 
cell typically produces an emf between its electrodes of 20 to 30 mV under 
lean exhaust conditions and 800 to 900 mV under rich conditions with a 
step like transition occurring near stoichiometry. 
Similarly, a publication by E. M. Logothetis, Ceramic Engineering Science 
Proceedings, 8th Automotive Materials Conference, 1, 281 (1980) describes 
a solid oxide (e.g. titanium dioxide) device whose resistance changes by 
several orders of magnitude at the stoichiometric A/F when it is 
alternately exposed to rich and lean exhaust gas conditions. This change 
in resistance is often determined using a bridge circuit in which the gas 
sensitive resistor in one arm of the bridge is used with a thermistor 
(whose temperature coefficient of resistance matches that of the A/F 
device but which is insensitive to the gas phase) in another appropriate 
arm of the bridge to compensate for any changes in resistance which occur 
due to temperature variations. Both of these stoichiometric A/F sensing 
principles can be embodied in a number of different materials. Because the 
electrochemical devices allow the possibility of oxygen pumping, a number 
of structures have used this process in combination with the measurement 
of the emf of the same or other cell (e.g. see U.S. Pat. No. 4,272,329 to 
Hetrick) in a method which enables the A/F measurement over a wide range 
of values. 
SUMMARY OF THE INVENTION 
This invention describes a device which accomplishes the stoichiometric A/F 
determination described above by measurement of a different physical 
parameter from those previously reported. In particular, this device 
measures the change in the workfunction of an appropriate surface as the 
chemical species in the gas phase adjacent to the surface, and in chemical 
interaction with the surface, make a corresponding change from net 
oxidizing to net reducing conditions. 
In one embodiment, the method by which the workfunction change is measured 
includes monitoring the gas-induced variation in the magnitude of the 
thermionically emitted, alkali metal ion (e.g. Na+) current coming from a 
heated emitting material whose surface (the surface whose workfunction is 
in question) is simultaneously capable of catalyzing the chemical reaction 
between the oxidizing and reducing gases. The magnitude of this 
thermionically emitted current is an especially advantageous parameter to 
measure since under appropriate conditions it is exponentially sensitive 
to the workfunction of the emitting surface. Platinum, which is a ready 
host material for low concentrations of alkali metal impurities that can 
be thermionically emitted, is an example of a material which is 
simultaneously an appropriate catalytic material. 
In the case of some materials the workfunction can change substantially 
(e.g. 1.0 V) due to the adsorption of gas phase species (e.g. oxygen). 
Further, when the material is exposed to a mixture of gases which will 
react catalytically through an adsorption mechanism, the workfunction may 
change significantly at the stoichiometric ratio of these reacting gases 
thus effecting a significant change in the thermionic current which thus 
serves to sense that ratio. Again Pt is a material where the above 
mentioned processes of catalysis, adsorption and thermionic emission are 
applicable. 
The sensing device includes a heated, catalytic material containing alkali 
metal (or other impurities suitable for the thermionic emission process) 
and held at a positive potential relative to a nearby collector electrode. 
Using appropriate electrical means, the thermionic current between the 
emitting and collecting electrodes is measured. As the result of varying 
conditions, the ratio of the concentrations of oxidizing and reducing 
species in the gas phase adjacent to the emitting surface is caused to 
vary about the stoichiometric ratio for these gases. Because the gases are 
catalytically reacting on the emitting surface by means of an adsorption 
mechanism, the work function of the surface is changing at the 
stoichiometric ratio resulting in a large, reversible change in the 
thermionic current which can thus be used to sense this ratio. The emitted 
impurity ions may also be held in a ceramic or other appropriate reservoir 
in contact with the emitter and supplying material for ionic emission to 
the emitter by diffusion.

DETAILED DESCRIPTION OF THE INVENTION 
Alkali metals such as Li, Na, K, and Cs are common impurities in many 
ceramic and metal materials. When these solids are heated, the volatile 
alkalis are thermally evaporated at modest temperatures. Thus, the alkalis 
have a low ionization potential (the ionization potential, IP, is 
proportional to the energy to remove the outermost, or least tightly 
bound, electron from an atom). Whether the thermal emission will occur as 
a neutral species (the thermal evaporation of atoms) or as charged species 
(the thermionic emission of positively charged ions) depends on the 
workfunction of the host solid. If the workfunction is large with respect 
to the IP of the atoms, it is energetically easy for the atom to leave its 
outermost electron on the metal and be thermionically emitted. Thus Cs 
with an IP of 3.9 V is often thermionically emitted while Li with an IP of 
5.8 V is most often thermally evaporated as an atom. Pt with a 
workfunction of about 5.0 V is a good host material for the thermionic 
emission of the alkali metals. 
These ideas are illustrated schematically in FIG. 1 which shows the solid 
workfunction with, wf(w), and without wf(wo), an oxide layer. This 
parameter is proportional to the energy required to remove the most 
energetic electron that is bound in the solid to the vacuum energy level 
where it could leave the solid if drawn away with an electric field for 
example. The energy scale is vertical in the drawing. IP is the comparable 
parameter for atoms or molecules and the values for two alkali metals 
(which have low values of IP) are shown. 
Electrons may transfer between atoms or molecules hitting the surface and 
the solid. Electrons would move from the species with the lowest binding 
energy to that with the greatest binding energy. As will be discussed 
further below, when the solid is exposed to an oxidizing gaseous ambient, 
the oxidizing species can interact with the surface and modify the 
workfunction (e.g. to a new value, wf(w), assuming that oxygen is the 
oxidizing species) thereby changing the rate of the thermionic emission. 
For electrons a high emission rate is promoted by a low workfunction; 
however, for positive ion emission as in the present case, the situation 
is more complicated and usually a high workfunction promotes stronger 
emission since it becomes easier for the surface impurity species (the 
alkali atom) to leave its outermost electron in the solid and 
thermionically emit as an ion rather than thermally evaporate as an atom. 
Solid state ion emitters based on this method are attractive since one 
avoids the complexity and cost of generating a gaseous plasma as an ion 
source. 
FIG. 2 shows the schematic diagram of a device in which a thin Pt emitter 
film 11 is tightly wrapped around a pencil-like ceramic heater 15 which 
has a region (that surrounded by the Pt) that can attain temperatures of 
800.degree. C. or higher. This emitter electrode is surrounded by a steel 
collector electrode 12 held at a distance of a few millimeters from the 
emitter. A battery 13 connected between the two electrodes holds the 
emitter at a positive potential (V.sub.EC) with respect to the collector 
while an ammeter 14 measures the magnitude of the thermionic current I 
(typically in the nA regime). Mass spectrographic studies indicated that 
ions 16 emitted from the heated (to greater than 400.degree. C. to achieve 
nA current levels) Pt where those of Na and K. When the emitting Pt is in 
the form of a sputtered film or a conducting metal-ceramic composite 
deposited on the heater, larger emitter currents can be obtained when the 
ceramic is doped with the alkali. The ceramic presumably supplied these 
atoms to the metal surfaces for thermionic emission by diffusion. 
FIG. 3a shows a typical plot of thermionic current versus emitter to 
collector voltage. When the emitter is negative relative to the collector, 
the current flow is very low (in the subpicoampere regime) while that with 
the opposite emitter bias shows a greater than linear increase in the nA 
regime. The values shown are typical for an operating temperature of 
650.degree. C. with these devices. In summary, the current-voltage 
characteristic is that of a thermionic diode in which the emitted carriers 
are of a positive sign and the conducting medium into which the ions are 
emitted is relatively resistive. In this case the resistance is provided 
by the collision of the positive ions with the neutral ambient gas phase 
(e.g. air) molecules which results in an ion mobility in the vicinity of 3 
cm.sup.2 /volt-sec at 100 kPa. Numerous results confirm this basic model. 
For example, if the I vs V.sub.EC characteristic is measured at reduced 
ambient pressure, the conductivity at positive emitter biases increases 
inversely with the pressure until a maximum current flow is encountered in 
the range of 0.01 kPa. This inverse dependence is consistent with the ion 
scattering mechanism mentioned above. At reduced pressure, the ion 
emission shows a saturation with increasingly positive emitter bias. 
Further, this saturation current (as with the thermionic current in all 
circumstances) increases with temperature in a manner consistent with 
thermionic emission. 
FIG. 3a also shows that the exposure of the emitting surface to a rich 
ambient causes a large reduction (as large as a factor of 100 depending on 
the temperature) in the emission at all emitter-collector voltages. The 
change from large to small currents occurs at the stoichiometric ratio of 
the oxidizing and reducing gases in the ambient. 
FIG. 3b shows a graphical representation of the alternating change in the 
emission current from high to low values as the A/F ratio is changed from 
lean (L) to rich (R) conditions respectively. Tests were run with a 
carrier gas of nitrogen (approximately 99%) with propane and oxygen 
(oxygen is at a 5:1 partial pressure excess over propane at stoichiometry) 
as the reducing and lean species. This effect is the basis of using the 
device as a stoichiometric A/F sensor. The changes shown in FIG. 3b are 
for changes in A/F in the immediate neighborhood of stoichiometry as would 
occur in a typical automotive combustion application where large 
departures from stoichiometry are usually not desirable. The emission is 
not constant for all lean or rich A/F but the changes are small, except 
for those occurring at stoichiometry, when the reactive gases are a small 
fraction (e.g.&lt;1%) of the total ambient concentration. 
FIG. 4 shows the temperature dependence of the rich and lean emission 
current at one atmosphere pressure (100 kPa) and V.sub.EC =10 V. This 
semilog plot shows that the ratio between lean and rich currents, which 
can be as high as 100 at the low temperature of 450.degree. C., becomes 
smaller as temperature increases with the two values eventually merging 
around 850.degree. C. This plot indicates that the emission process is 
thermally activated and that the activation energy is lower under lean 
ambient conditions. This result is reasonable in the likely possibility 
that that activation energy for thermionic emission is in part determined 
by the workfunction of the Pt in a manner such that a larger workfunction 
(which makes it energetically easier for an adsorbed alkali atom to 
transfer its electron to the Pt) corresponds to a smaller activation 
energy. It is well known that lean or oxidizing ambient conditions can 
increase the workfunction of many metals by as much a 1 V. This can arise 
when adsorbed oxygen attracts metal electrons leading to a surface double 
layer which is of the right sign to increase the workfunction. 
If on passing to a rich ambient, reducing species react with and remove the 
adsorbed oxygen, then a corresponding reduction of the workfunction is 
indicated with a corresponding increase in the activation energy (and 
decrease in current) for the thermionic emission of adsorbed alkalis. In 
summary, it appears that the A/F ratio sensing phenomena of this work is 
one in which the gas phase modulates the workfunction of an appropriate 
material and accordingly the rate of thermionic emission of alkali dopants 
in the structure is also modulated. 
Various modifications and variations will no doubt occur to those skilled 
in the various arts to which this invention pertains. For example, the 
geometric configuration of the sensor structure need not be cylindrical 
but might advantageously have a planar geometry. These and all other 
variations which basically rely on the teachings through which this 
disclosure has advanced the state of the art are properly considered 
within the scope of this invention.