Note: Descriptions are shown in the official language in which they were submitted.
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STEADY-SrATE METHOD F9R DETERMINING RICH AIR/FUEL RATIOS
This invention relates to determining ~he compo-
sition of a gaseous atmosphere.
An important application of high temperature ya5
sensors is in the determination of the air to ~uel ratio
(A/F = mass of air/mass of fuel) in the exhaus~ gases of
hydrocarbon fired furnaces or engines such as an automo-
bile internal combustion engine. The stoichiometric A/F
is one in which the mass of air present contains just
enough oxygen (2~ to react with the hydrocarbons (HC)
present so that there is the minimum amount of both 2 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
s~bstantial 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 temperaturo oxyg~n 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; C9. At thermodynamic equilibrium, the
concentrations of these species increase .~onotonically
with decreasing A/F and thereby provide a measure of rich
A/F-
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High temperature oxygen sensors using electro-
chemical cells fabricated from the cera~nic solid electro-
lyte zirconium dioxide, ZrO2, doped with Y203 (or 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 2
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 2 partial pressure adjacent to
the inner electrode is given by PREF- PEX~ the 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) ln (PREF/pEx) (1)
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
PEX because of the logarithmic function. This disadvan-
tage is offset near the stoichiometric A/F because PEX canchange 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~
~;~09;Z~9
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One method of enhancing sensitivity is through
the use f 2 pumping devices also employing ZrO~
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 2 concentra-
tion at higher sensitivity. In the case oE 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 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 Ximura 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
2 is pumped at a high enough rate from an 2 "reservoir"
side of the cell to the l'reaction" side of the cell to
bring the 2 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.
Z~
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However, such single cell devices may be subject
to significant }oss of calibration (drifting~ or deterior-
ation with extended use as would be required i~ automotive
applications. The voltage across the pump cell, which for
these devices is ~ 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 2 is passed ~hrough 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. Purther, the ohmic contribution to the
voltage across the cell will vary exponentially with
temperature requiring tight temperature control with
probable penaltie~ in cost and performance.
On the other hand, ~ith 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.
In accordance with an aspect of the present
invention, there is provided a method of making a measure-
ment of air to fuel ratio rich of the stoichiometric value
in an ambient environment containing unreacted and par-
tially reacted hydrocarbons including the steps of estab-
lishing an enclosed volume with restricted access to the
ambient environment by means of supporting a first elec-
trochemical cell and a second electrochemical cell, at
spaced positions and defining therebetween a volume;
substantially electrically isolating the first electro-
chemical cell from the second electrochemical cell so that
there is a substantially reduced direct electrical coup-
ling between a voltage. across the first electrochemical
cell and a voltage across the second electrochemical cell;
~2~926~9
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providing communication from the ambient environment to
the volume through an opening between the volume and the
ambient; establishing a reference oxygen partial pressure
having a higher oxygen partial pressure than the ambien-t
environment and positioning a barrier between the ambient
5 environment and the reference oxygen partial pressure so
as to physically isolate the ambient environment from the
;reference oxygen partial pressure; exposing a first side
of the first electrochemical cell to the volume; and
exposing a first side of the second electrochemical cell
lO to the volume and a second side to a reference oxygen
partial pressure.
In accordance with another aspec-t of the present
invention, there is provided a method of making a measure-
ment of air to fuel (A/F) ratio rich of the stoichiometric
15 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
:20 spaced positions and defining therebetween a volume, each
oF the first and second electrochemical cells having
:opposing electrodes; providi.ng communication by means of
gaseous diffusion from the ambient environment to the
volume through an opening between the volume and the
25 ambient; exposing a first side of the first electrochem-
ical cell and a first side of the second electrochemical
cell to the volume; establishing a reference oxygen par-
tial pressure, having a higher oxygen partial pressure
than the ambient environment adjacent to a second side of
30 the second electrochemical cell; exposing a second side of
the first electrochemical cell to the ambient environ-
ment; passing a pump current through the second electro-
chemical cell so that oxygen is pumped from the reference
atmosphere into the enclosed volume and so that, by chem-
35 ical reaction of the oxygen with the unreacted and par-
tially reacted hydrocarbons within the volume, a differ-
ence in oxygen partial pressure is established at t.he
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first electrochemical cell thereby causing an EMF to be
generated b~tween the opposing electrodes of the first
electrochemical cell; coupling the first and second elec
trochemical cells together in a ~eedback control manner,
and providing an amount of pump curren~ to -the second
electrochemical cell so that the EMF induced across -the
first electrochemical cell is maintained at a cons~ant
reference voltage value, the coupling between the first
and second electrochemical cells having a sufficiently
high impedance to substantially electrically isolate a
voltage applied across the second electrochemical cell
from a voltage sensed across the fi.rst electrochemical
cell; establishing and maintaining the reference voltage
value in an external circuit coupled to the ~irst electro-
chemical cell; measuring the magni~ude of the pump cur-
rent; and determining the rich A/F ratio from the magni-
tude of the pump current.
The invention is described further, by way of
illustration, with reference to the accompanying drawings,
in which:
20Figure 1 is a schematic cross section of a
sensor structure ~or making rich A/F measurem~nts in
accordance with an embodiment of the steady-state pumping
method of this invention;
FigO 2 is a graphic representation of sensor cell
voltage, Vs, versus p~mp cell current Ip at various rich
A/F values for the sensor structure shown in Fig. l;
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 ~or various A/F values in
acco~dance with the sensor structure shown in Fig. l; and
Fig. 4 is a sche~atic 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.
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Referring to Fig. 1, an air fuel sensor 110
includes an electrochemical cell 111 including a disk-like
electrolyte 11~ of a solid ionic conductor o~ oxygen such
as Y203 doped ZrO2. 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 lZ4.
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 es~ablish 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 e~posing one side
of cell 121 ~o a reference atmosphere. As a result, one
side of the sensor is exposed to the exhaust gas and one
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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 2 to be pumped into v by cell 121 tpump 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 vsl 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 in this case,
Vs = (RT/4F) ln (PY/pEx)~ (2)
Fig. 2 shows a plot of induced EMF, vsl versus
pump current, Ip, at different rich air fuel ratio values.
The EMF is low for small pump currents and increases with
2~921)~
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 oE the Ip required to cause the EMF on electro-
chemical cell 111 to reach an arbitrary reference valueV(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 Rl, 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
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g
structure of Figs. 1 and 4 is further discussed in appli-
cant's copending Canadian patent application Serial No~
456,727 filed June 15,1984.
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.
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