Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 --
EXTENDED RANGE AIR FUEL RATIO SWISS
This invention relates to determining the combo-
session of a gaseous atmosphere.
In the following description, reference is made
to the accompanying drawings, wherein:
Figs. l, 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;
It Fig 5 is a graphic representation or the sensor
cell voltage, By 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, VA, versus pump cell current, It, at variolls rich
lo air fuel values for a sensor in accordance with Fig. 4;
Fig. 7 is a graphic representation of the pump
cell current, It,' required to hold the voltage of the
sensor cell at a reference voltage for various air fuel
ratios, A/F, in accordance with the sensor of Fig 4;
Fly. 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
measure in A/F; and
Fig. 9 is a schematic cross section of a sensor
I in accordance with the second embodiment of this
invention
It is known to use high temperature oxygen
sensors in the determination of a s~oichiometric air fuel
mixture in the exhaust gases of automobile internal combs-
lion engines. The stoichiometric mixture is one in which
the mast of air present contains just enough oxygen to
react with the mass of hydrocarbons present so that there
it the minimum amount of both oxygen and hydrocarbons
remaining. The air fuel ratio I = mass of air/mass of
I fuel) at the stoichiometric point is approxim tell 14.6.
If, for example, an engine were running lean of statue-
metro (A/F > 14.6), there would be an excess of air in the
"charge" burned in the cylinder of an internal combustion
- lo
engine and the exhaust gas would contain a substantial
oxygen partial pressure. of rich operation were occurring
(A/F < 14.6), the exhaust gas would contain unrequited 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 stoichiometeric air fuel ratio with an
exhaust gas oxygen sensor. The electrical output of such
a sensor can then be fed back to an electrically
I
controllable carburetor or fuel injection system Jo,
maintaining engine operation at the stoichiometric point.
Depending on engine type, operation a 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 (EM) proportional to the natural logarithm
of the oxygen partial pressure. Despite their low sense-
tivity to oxygen partial pressure, the large change in
oxygen partial pressure at the stoichiometric point allows
their useful implementation. US. Patents 3,948,081;
3,738,341; 4,112,893; 4,21~,509; and 4,107,019 relate to
oxygen sensors of this type.
US. Patent Nos. 3,907,657 to Hygiene and
3,514,377 to Spicily et at relate to the measurement of
oxygen (2) concentrations using solid electrochemical
devices. For applications at elevated temperatures
( > 500 I 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. Electron
chemical cells made from this material are suitable at
elevated temperatures for oxygen sensing and pumping
applications.
The mode of operation of the Hygiene device can be
described as an oxygen counting mode in which oxygen
partial pressure is determined on a sampling basis. A
I
constant current is applied to an electrochemical cull
which forms part of the enclosure of a volume for a period
of time to, for the purpose of electrochemically pumping
out most of the oxygen from that volume. The ambient
atmosphere is established within the volume prior to the
pump out by means of a leak. An additional electron
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 Hygiene). Knowing the tempera-
lure, enclosed volume, pump out current and time allows
one to calculate the number of oxygen molecules within the
enclosure from the ideal gas law. The number of oxygen
molecules is in turn proportional to the desired oxygen
partial pressure. If a constant pump current is used, the
pump out time to is proportional to the oxygen partial
pressure. If a constant current is not used, then the
integral of the pump out current over the pump out time is
proportional to the oxygen partial pressure.
The ~eijne device can provide an output which is
linearly proportional to the oxygen partial pressure.
This is superior, for example, to single oxygen concentra-
lion cells used as sensors which give an output (EM)
proportional to the natural logarithm of the oxygen
partial pressure in (Pox
A potential disadvantage of the Hygiene 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 signify-
cant amount of oxygen leaks into the volume to cause an
--4--
error in the count of molecules (i.e., to erroneous
increase to ). However, if the leak is made small, it
may take a long time, TV 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 US. Patent No.
3,698,384 to Jones, the purpose is to measure oxygen
partial pressure in a edgewise. This is done by measuring
an electrochemical cell pumping current while holding the
sensor cell voltage a constant. However, the flow rate of
the fudges 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 employs 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 tonically conducting zip-
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
US. Patent No. 3,907,657, the operation is different.
Here one applies a pump voltage V to remove oxygen from an
--5--
enclosed volume 6 until the pump current saturates. one
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
equals the pump current times a proportionality constant.
The current saturates because the leak aperture in combine-
lion 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 sonnetizing 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 US. Patent
Nos. 4,272,331 and 4,272,330 to R. E. He trick and US.
Patent No. 4,272~329 to I E. trick et at. The sensors
(shown as prior art in Fig. 3 of the drawings) are placed
entirely in the exhaust gas stream and include two oxygen
--6--
ion conducting electrochemical cells 11 and 12, a pup
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 ox
the tube by ceramic glue 16. A small aperture 17 in toe
enclosing structure allows the exhaust gases, containing
oxygen in a percentage to be determined, to leak into the
volume. Lead wires I are affixed to electrodes 15
attached to each side of electrochemical cells 11 and 12.
The He trick and He trick et at patents describe
various external circuitry which can be coupled to the
sensors to permit operation in modes including an oscil-
lottery mode, a transient mode, and a steady-state mode.
When operated in one of these modes t 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, It. 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 EM (= Us) 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 It and Vs.
Further, in addition to the previously discussed
stoichiometric and lean aureole operation, there are
occasions where engine operation rich of stoichiometry is
--7--
desired. In this region, the amount of partially reacted
hydrocarbons (HO) such as carbon monoxide and n~drogen
which increase with decreasing A/F can serve as a measure
of A/F. Using oxygen pumping cells 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 HO.
Thus, US. Patents 4,224,113 and 4,169,440
describe sinlessly 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
25 may vary with time as the electrode stinters 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
30 cost and performance. An advantage of two-cell structures
such as those described by the He trick and He trick et at.
patents is that the pump-cell potential difference is not
a critical parameter thereby lessening the effects of
electrode deterioration and temperature.
I
Thus it con be appreciated that die rent errs
structures and different external circuitry are esp~iall~
advantageous for I measurements in particular limited
A/F regions. Current sensors which apply to a broader
5 range may not possess the desirable features associate
with the devices covering a more limited range. In any
case, it would by 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 statue-
lo metro, which incorporate the most useful properties These are some of the problems this invention overcomes.
In accordance with one aspect of the present
invention, there is provided an air fuel sensor for mews-
using an extended range of air fuel ratios both rich and
lean of stoichiometry, the sensor including a first elect
trochemical cell; a second electrochemical cell; a coup-
lying means for attaching the first and second electron
chemical cells to one another at spaced positions and
defining there between a volume, the coupling means prove-
20 ding a relatively high impedance coupling between the first and second electrochemical cells so that a first
voltage across the first electrochemical cell can vary
relatively independently of a second voltage across the
second electrochemical cell, and the coupling means prove-
25 ding for cooperation between the first electrochemicalcell and the second electrochemical cell in a feedback
manner; the first electrochemical cell having a first side
exposed to the volume and a second side exposed to a
sample ambient gas; the air fuel sensor having an opening
30 there through so as to provide communication between -the
volume and the sample ambient gas; and the second electron
chemical cell having a first side exposed to the volume
and a second side exposed to a reference oxygen partial
pressure having an oxygen partial pressure greater than
35 that of the sample ambient gas.
In accordance with another aspect of the present
invention, there is provided an exhaust gas oxygen sensor
Lo
g
for generating a signal indicative of the air fuel Rio
of operation of an internal combustion engine generating
exhaust gas including a first and a second electrochemical
cell spaced from one another and defining there between a
partially enclosed volume, the volume having communique-
lion with the exhaust gases through an opening, and a
first side of each of the first and second electrochemical
cells being exposed to the volume; a coupling means for
supporting the first and second electrochemical cells, for
lo providing a relatively high impedance coupling between the
first and second electrochemical cells so that a first
voltage across the first eleçtrochemical cell can vary
relatively independently of a second voltage across the
second electrochemical cell, and for providing for cooper-
15 anion between the first electrochemical cell and the second electrochemical cell in a feedback manner; a second
side of the first electrochemical cell being exposed to
the exhaust gases; a second side of the second electron
chemical cell being exposed to a reference ambient atom-
20 sphere; and each of the electrochemical cells having first electrode on each of the first sides and a second
electrode on each of the second sides.
In accordance with a further aspect of the
present invention, there is provided an exhaust gas oxygen
25 sensor for generating a signal indicative of the air fuel
ratio of operation of an internal combustion engine goner-
cling exhaust gas, the exhaust gas oxygen sensor being
positioned within the exhaust gas and including a first
and a second electrochemical cell; a coupling means for
attaching the first and second electrochemical cells to
one another at spaced apart positions and defining there-
between a partially enclosed volume, the volume having
communication with the exhaust gases through an opening,
and a first side of each of the first and second electron
chemical cells being exposed to the volume; the coupling means providing a relatively high impudence coupling
between the first and second electrochemical cells so that
a first voltage across tile first electrochernical cell can
- pa -
vary relatively independently ox a second voltage across
the second electrochemical cell, and the coupling means
prodding for cooperation between the first electrochem-
ical-cell and the second electrochemical cell in a feed-
back manner; a second side of the first electrochemicalcell being exposed to the exhaust gases; and a second side
of the second electrochemical cell being exposed to a
metal/metal oxide compound which acts as a reference
oxygen partial pressure.
A device in accordance with this invention can
- 10 -
be used it different measurement techniques to dormancy
exhaust gas A/F over a wide range ox values including
those richer than, leaner than, and near the statue-
metric air fuel value. Hence, the device has a "uniter-
set" air fuel sensing characteristic. Further, tune cell structure allows the use of measurement techniques which
are particularly advantageous in each of the three ranges.
referring Jo 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 Yo-yo doped ZrO2. Cell 111 also includes two thin
porous catalytic platinum electrodes 113 with attached
lead wire 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
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 12~ is provided 'o en
A/F sensor 110 within a desired operating temperature
range.
Referring to Fig. 9, another embodiment in accord
dance with this invention replaces the air reverence on-
one side of cell 121 of Fig. 4 with a metal, metai-oxide
mixture. Air fuel sensor 140 of Fig. 9 has an electron
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 Jo 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 post-
toned 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-
vises 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 slot biometric air fuel
value. Hence, the device Jan be considered to have
"universal" sensing characteristics. First, a
steady state oxygen-pumping mode is used for an extended
12~
rang of rich and lean air fuel ratio values. Second, the
previously described single electrocheMica1 cell technique
is used near stoichiometryO The structure of the device
of Fig. 4 permits use of multiple measurement techniques
Jo that the functional advantages of each technique can be
realized in a particular air fuel ratio region of applique-
lion. The use of the single electrochemical cell for
measuring A/F near stoichiometry is well known and is
taught in US. Patent 3,948,081 to Weasel et at. The use
of pumping techniques for lean A/F sensing using a two
cell structure are taught by Us Patent 4,272,329 to
Heroic et at and US. Patents 4,272,330 and 4,272,331 to
He trick. The use of two-cell pumping techniques for rich
A/F sensing are further discussed in our cop ending apply-
cation Serial No. 455,788 filed June 4, 1984 entitled
Steadiest Method of Determining Rich Air/Fuel Ratios".
Referring to Fig. 4, hollow spacer 125 is shown
as being a generally insulating material and thus provides
a relatively high impedance coupling between electrochem-
teal cell 111 and electrochemical cell 121. Accordingly,
the voltage across the two electrochemical cells 111 and
121 can vary relatively independently and the two electron
chemical cells 111 and 121 can cooperate in a feedback
manner. For example, the feedback manner of electrochem-
teal cells 111 and 121 is controlled by associated air-
quoter such as shown in Fig. 8.
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 singly electrochemical cell sensor
previously described in connection with sensing
stoichiometric air fuel ratios The equilibrium oxygen
partial pressure for the exhaust gas Pox, is established
at the catalytic electrode 1~3 ox cell 121 within volume
v. In combination with the oxygen partial pressure in the
~12~
air reference, PUFF being equal to 0. 2 atmospheres, on
EM, VB7 is generated across cell 121 given by the Ernst
equation:
VB = (RT/4F) I (PREF/PEX) (1)
-13~ 4~Z
where R is the gas constant, T is the absolute temperature
and F is the Faraday constant. As POX and the cores-
pounding air fuel ratio decrease! the cell EM increases as
shown in Fig. 5. The strong variation of exhaust gas
oxygen and, correspondingly, the cell EM 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 EM 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 US.
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 EM
induced on the other "sensor" cell, erg. 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 It and the
"sensor" cell EM 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 It to be
passed through the pump cell 121 to withdraw just enough
oxygen from v so that a constant sensor cell EM, termed
Us, is established. As the percentage of oxygen
increases, so does the required pump current thereby
providing a measure of the oxygen percentage and cores-
pounding A/F. In particular, one finds that
It = POX (1 - e ~Vs/vo) (2)
-14
where VOW = RK/4F and G is a constant of proportional
defining the rate at which oxygen can diffuse into v
through the leak aperture. For example, G is
proportional to the oxygen diffusion coefficient and the
area of the leak aperture. Thus, by always passing just
enough It to keep Us a constant, It POX 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-
lion 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 what 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, It, 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.
During operation with air fuel ratios rich of
stoichiometry (A/F 14.7), the concentrations of
partially reacted I 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 It.
-15-
Simultaneously, the oxygen partial pressure within v is
decreased by oxygen diffusion through leak aperture 126
and chemical reaction of interior catalytic electrodes 123
and 113 with the partially reacted I which continuously
diffuses into volume v through leak aperture 126.
As pump cell current It increases, the
equilibrium oxygen partial pressure within volume v
increases causing an EM to be induced across
electrochemical cell 111. The magnitude of this EM,
termed VIA is again given by Equation 1 where PREY is
replaced by Pi which represents the near equilibrium
oxygen partial pressure within volume v resulting from the
reaction of pumped oxygen and partially reacted Ho Since
Pi > POX in this case, the sign of the EM will be
opposite that induced by pumping action during lean air
fuel ratio measurement.
Figure 6 shows a plot of induced EM VA, versus
pump current, It, at different rich air fuel ratio values.
The EM is low for small pump currents and increases with
It. For lower air fuel ratio an ever increasing amount of
oxygen must be pumped into volume v to accomplish a
significant reaction with the HO In particular, the
value of It required to cause the EM on electrochemical
cell 111 to reach an arbitrary reference value VERIFY)
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 VERIFY) 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 It will be an increasing function of cell
volume and leak aperture size.
-16~
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. A circuit
similar to the one shown schematically in Fig. 8 would be
S attached to both cells. In Fig. 8, the supporting
structure is not shown for clarity. Resistors Al, R2 and
capacitor C control the gain and frequency response of
amplifier A so that A will always generate enough pump
current It to maintain the Elf across cell 111 at a
constant value equal to VERIFY). A resistor I is
included in the pump cell circuit so that It can be
determined by measuring the voltage across R3 with
voltmeter V. Using the calibration curves of Fig. 6, the
air fuel ratio would be determined. Using standard
electronic circuitry this current can be compared to the
value of It required for a desired air fuel ratio. If the
current is too high or low, intake fuel could be increased
or decreased, respectively, thereby accomplishing 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 It to a
temperature compensated value if necessary.
In summary, operation rich or lean of
stoichiometry would be accomplished by pumping oxygen into
or out of the enclosed volume until a predetermined VA
OF appropriate for rich or lean conditions is achieved.
With rich and lean calibration curves electronically avail-
able as in an inboard 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.
-17~
Because of the highly exothermic nature of Tao
HC-oxygen reaction, very small amounts of pumped oxygen
can cause wide variations in VA 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 operatillg results. As an example, one
finds in the steady state mode that
P APEX PROOF evasive) (3)
Thus, by adjusting It to keep Us always fixed PROOF is
assumed to be constant) at an arbitrary value, It is still
proportional to POX although offset by a constant amount
from the value found in Equation (2). A judicious choice
of Us will still allow convenient lean operation with high
sensitivity.
-18-
The advantage of this reversal of pump and sensor
cells would be to eliminate current flow in cell 121 icon
may also be used as the sensor cell in subsequent
stoichiometric operation It is known that if current
flow is too large in oxygen ion conductors, electrolyte or
electrode deterioration can occur. This in turn could
cause false or spurious Ems to develop under open
circuit conditions so that subsequent operation as a
sensor cell would be compromised. In this case, however,
lo 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. Nina, Quick).
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 lSl. 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, Ammo (REV), is much less than
Pox under lean conditions, a substantial EM (e.g. 200-500
my) will appear across the sensor cell 145 at It = JO As
oxygen is pumped from volume v9 by pump cell 141, this EM
will be reduced. Choosing an appropriate EM in this
reduced range as a reference value, analysis analogous to
that used in US. 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
-1 I
in the previously discussed cases. For near statue-
metric operation the pump cell 141 is disconnected and the
open circuit EM 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 EM which is adequate to determine air fuel
ratios in a narrow range. pumping mode for rich air
fuel ratio detection requires that sufficient oxygen be
available.
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
15 through which this disclosure has advanced the art are
properly considered within the scope of this invention.
