U.S. patent number 3,893,230 [Application Number 05/391,424] was granted by the patent office on 1975-07-08 for method of manufacture of an exhaust gas sensor for an air-fuel ratio sensing system.
This patent grant is currently assigned to Ford Motor Company. Invention is credited to Michael J. Esper, Donald J. Romine, Henry L. Stadler, Tseng-Ying Tien.
United States Patent |
3,893,230 |
Stadler , et al. |
July 8, 1975 |
Method of manufacture of an exhaust gas sensor for an air-fuel
ratio sensing system
Abstract
A titanium dioxide ceramic disc having two spaced electrodes
embedded therein is located in the exhaust passage of a
reciprocating internal combustion engine. The electrical resistance
across the electrodes is proportional to the equilibrium oxygen
pressure of the exhaust gases and thus represents the air-fuel
ratio of the mixture supplied to the engine.
Inventors: |
Stadler; Henry L. (Ann Arbor,
MI), Tien; Tseng-Ying (Ann Arbor, MI), Esper; Michael
J. (Detroit, MI), Romine; Donald J. (Southfield,
MI) |
Assignee: |
Ford Motor Company (Dearborn,
MI)
|
Family
ID: |
26893854 |
Appl.
No.: |
05/391,424 |
Filed: |
August 23, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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198515 |
Nov 15, 1971 |
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Current U.S.
Class: |
29/592.1;
422/105; 436/143; 156/89.16 |
Current CPC
Class: |
F02D
41/1455 (20130101); G01N 27/12 (20130101); F02D
41/30 (20130101); F02D 9/00 (20130101); C04B
35/632 (20130101); C04B 35/46 (20130101); Y10T
436/218 (20150115); Y10T 29/49002 (20150115); F02D
2700/09 (20130101); G01N 2291/02863 (20130101) |
Current International
Class: |
G01N
27/12 (20060101); C04B 35/632 (20060101); F02D
41/30 (20060101); C04B 35/63 (20060101); C04B
35/46 (20060101); F02D 41/14 (20060101); F02D
9/00 (20060101); G01n 029/02 () |
Field of
Search: |
;23/232E,254E,255E
;204/195P,195M,195S ;73/27R,26 ;338/34,249,254,255,256,314
;29/592,610,612,613,614,625,627 ;156/89,228,242,246 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lanham; C. W.
Assistant Examiner: Walkowski; Joseph A.
Attorney, Agent or Firm: Benziger; Robert A. Zerschling;
Keith L.
Parent Case Text
This is a continuation of application Ser. No. 198,515, filed Nov.
15, 1971, now abandoned.
Claims
We claim:
1. A process for preparing an exhaust gas sensing member comprising
the steps of
forming a slurry of a major portion of particles of a transition
metal oxide;
drying the slurry into a sheet;
forming plates from the dried slurry;
sandwiching spaced electrodes between a pair of said plates;
and
firing the sandwiched plates into a unitary structure.
2. The process of claim 1 including the step of sandwiching an
electrical heating means between a third plate and one of said pair
of plates prior to the step of firing.
3. The process of claim 1 wherein the slurry is formed of particles
of the transition metal oxide capable of being fired to achieve a
density of from about 80% of theoretical to about 95% of
theoretical.
4. The process of claim 3 wherein the slurry is formed of -325 mesh
particles of the transition metal oxide.
5. The process of claim 3 wherein the sandwiched plates are fired
at a temperature of from about 2300.degree.F to about 2500.degree.F
for a period of time less than about two hours.
6. The process of claim 1 wherein the step of sandwiching comprises
the steps of: placing a pair of plates in confronting relation on
opposite
sides of a pair of spaced electrodes; and pressing said plates
together under a pressure of about 10,000 psi.
Description
SUMMARY OF THE INVENTION
The temperature of the exhaust gases leaving the combustion
chambers of reciprocating internal combustion engines is
proportional to the amount of combustion taking place within the
engine and this relationship has been used in aircraft for
indicating the air-fuel ratio of the combustible mixture being
supplied to the engine. Subsequent investigations showed that the
thermal conductivities of various exhaust gas components could be
used to indicate the proportion of such components in the exhaust
gases. These investigations produced systems of the resistance
bridge type that compared the thermal conductivity of the exhaust
gases with known gas mixtures to indicate either air-fuel ratio or
the combustion efficiency of the engine.
Recent interest in improving the environment by diminishing the
quantity of undesirable components in the exhaust gases of
automotove engines has accentuated investigations into systems for
monitoring continuously the air-fuel ratio of combustible mixtures.
These investigations have led to numerous refinements of the
thermal conductivity system. For example, it was found that thermal
conductivity varies almost linearly with the carbon dioxide content
of the exhaust gases and carbon dioxide content in turn is
proportional to the air-fuel ratio. Subsequently it was found that
the thermal conductivity of the exhaust gases is a function of both
the carbon dioxide content and the hydrogen content. Other
approaches involved combining thermal conductivity devices with
exhaust gas temperature devices.
This invention provides a system for determining the air-fuel ratio
of the mixture supplied to a combustion mechanism by detecting
directly the oxidation-reduction characteristics of the exhaust
gases. The system comprises a sensing member made of a metal
compound containing oxygen atoms and having at least two metal
oxidation states of approximately equal energies that is located in
contact with either the air-fuel mixture supplied to the combustion
mechanism or the exhaust gases leaving the mechanism. Two
electrodes spaced apart from each other by at least a portion of
the sensing member are attached to the member and to an electrical
or electronic device for sensing the electrical resistance across
the electrodes. The electrical resistance is proportional to the
equilibrium oxygen pressure of the gaseous mixture in contact
therewith and resistance measurements can be converted directly
into the air-fuel ratio of the mixture supplied to the combustion
mechanism.
Equilibrium oxygen pressure is the partial pressure of the oxygen
in a gaseous mixture when the mixture is brought to complete
chemical equilibrium. The system of this invention measures
equilibrium oxygen pressure of a gaseous mixture even though the
gaseous mixture is not at chemical equilibrium, i.e., even though
the actual partial pressure of the oxygen exceeds the partial
pressure that would be present at equilibrium.
Useful metal compounds containing oxygen atoms and having at least
two oxidation states of the metal of approximately equal energies
include transition metal oxides such as titanium dioxide, vanadium
oxide, chromium oxide, manganese oxide, iron oxide, nickel oxide,
cobalt oxide, and rare earth metal oxides such as cerium oxide,
praseodymium oxide, etc. Oxides of the metals are preferable
because the ceramic properties thereof provide relatively long
useful lives at higher operating temperatures and of the inherent
presence of oxygen atoms. Other compounds and mixtures of the
oxides with each other and with the other compounds also can be
used. Energies of the two oxidation states of the metals must be
sufficiently close to permit reversal by changes in the equilibrium
oxygen pressure of the gases at operating temperature. Simple
empirical tests can be used to determine the required
relationship.
Sensing members made from the metal compounds preferably are
located in the exhaust gases leaving the combustion mechanism
because the exhaust gases approximate more closely the desired
operating temperatures of the members and do not contain any
unvaporized fuel. The system of the invention is useful
particularly in measuring and controlling the air-fuel ratio of the
combustible mixture being supplied to an internal combustion
engine.
The sensing member preferably is a relatively thin plate made from
sintered particles of the desired metal compound. Such plates
typically have a density of about 80-95 percent of theoretical;
plates of lower density operate effectively but tend to be
frangible while plates of higher density, including theoretical
density, exhibit decreasing response times. Particle size affects
response time only; a wide variety of particles sizes can be used.
The electrodes are attached to a surface of the plate or embedded
within the plate. One preferred construction involves sandwiching
the electrodes between two green ceramic plates and firing the
assembly into a unitary structure.
Maintaining the sensing member within a relatively broad
temperature range, typically about 600.degree.-900.degree.C.,
produces adequate indications of the air-fuel ratio supplied to an
engine despite the fact that temperature variations change the
resistance between the electrodes. Temperatures below 600.degree.C
tend to coat the member with soot and other particulate impurities
while temperatures above 900.degree.C tend to decrease overall
life. Accuracy improvements are achieved by associating a
controlled electrical heater with the sensing member to maintain
its temperature within a narrower range. A highly useful structure
involves a sandwich made of three green ceramic plates with the
electrodes between an outer plate and the middle plate and an
electrical resistance wire between the middle plate and the other
outer plate. A thermocouple for temperature control can be embedded
with either the electrodes or the resistance wire.
It is believed that the metal ions of the metal compounds are
reduced or oxidized from one oxidation state to the other in
proportion to the reducing or oxidizing nature of the exhaust
gases. In the case of titanium dioxide molecules, for example,
reduction frees an electron that conducts current much more readily
and thereby reduces the resistance of the portion of the ceramic
material located between the electrical leads.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a reciprocating internal
combustion engine showing the installation in the exhaust pipe of a
sensing member of this invention.
FIG. 2 is a schematic plan view of a disc-shaped sensing member of
this invention showing the electrical connections thereto.
FIG. 3 is a sectional elevation of the disc-shaped sensing member
of FIG. 2 showing the disposition of the electrodes, an electrical
resistance heating wire, and a thermocouple in a sandwich
construction.
DETAILED DESCRIPTION
Referring to FIG. 1, a reciprocating internal combustion engine 10
includes an intake manifold 12 for delivering an air-fuel mixture
to the engine combustion chambers (not shown) and an exhaust
manifold 14 for removing the combustion products from the
combustion chambers. A carburetor 16 is attached to the intake
manifold and an air cleaner 18 is attached to the air inlet of
carburetor 16. Carburetor 16 receives fuel from a fuel source (not
shown), produces an air-fuel mixture and supplies the air-fuel
mixture to intake manifold 12.
Exhaust manifold 14 is connected to an exhaust pipe 20. Threaded
into the wall of exhuast pipe 20 is a plug-shaped member 22
comprising a disc-shaped ceramic sensing member 24. Three sets of
electrical leads 26, 27 and 28 extend from the top of plug-shaped
member 22.
Turning to FIGS. 2 and 3, sensing member 22 comprises a sandwich of
three thin ceramic plates 30, 32 and 34. A length of electrical
resistance wire 36 is sandwiched between plates 30 and 32. Two
electrodes 38 and 40 and a thermocouple 42 are sandwiched between
plates 32 and 34. Electrodes 38 and 40 are spaced apart a
considerable distance as shown in FIG. 2. The entire sandwich is
fired into a unitary structure by conventional ceramic firing
techniques.
Electrical leads 26 connect electrodes 38 and 40 to an electrical
resistance sensor 44 as shown in FIG. 2. Leads 27 connect the ends
of resistance wire 36 to an electrical power supply 46 and leads 28
connect thermocouple 42 to a control circuit 48 located between the
power supply and one end of the resistance wire.
Each of plates 30, 32 and 34 consists essentially of titanium
dioxide. Each plate has a final thickness of about 0.008 inch and a
diameter of about 0.25 inch. The plates are made by a cast tape
process that comprises casting a titanium dioxide slurry onto a
plastic carrier tape, evaporating the vehicle from the slurry,
stripping the plastic tape and punching discs from the remaining
green ceramic layer. Titanium dioxide powder having a particle size
of -325 mesh is used to make the slurry although subsequent
grinding steps might reduce the final particle size.
Resistance wire 36 typically is made of platinum alloyed with about
13% rhodium and is about 0.008 inches in diameter. Electrodes 38
and 40 typically are made of platinum and are about 0.008 inches in
diameter. Thermocouple 42 is a gold-palladium-platinum and
gold-palladium combination. The green plates, resistance wire,
electrodes and thermocouple are sandwiched together as shown and
pressed at about 10,000 psi. After pressing, the assembly is fired
at about 2300.degree.-2500.degree.F. for 1-2 hours.
The resulting disc is installed in the exhaust conduit of a
reciprocating internal combustion engine where exhaust gases will
heat the disc to about 700.degree.C. When the engine is supplied
with an air-fuel mixture of about 13:1, the resistance across the
electrodes is about 2000 ohms. Changing the air-fuel ratio to 14:1
without changing any other engine parameters increases the
resistance to about 7000 ohms. An air-fuel ratio of 15:1 produces a
resistance of 20,000 ohms. Other embodiments produce resistances
ranging from 100 ohms at an air-fuel ratio of 11:1 to 500,000 ohms
at a ratio of 16:1.
Actual values of electrical resistance of course depend also on the
distance between the electrodes and temperature, but these factors
shift the entire resistance vs. air-fuel ratio curve without
affecting significantly the shape of the curve. Resistance changes
rapidly in the vicinity of the stoichiometric air-fuel ratios and
considerable temperature variations can be tolerated when
measurements are being made in that vicinity.
A wide variety of materials can be used to make the elctrodes,
resistance wire and thermocouple used in the sensing member. The
sensing member also can be formed in a wide variety of sizes and
shapes including cylinders, squares, rectangles, etc.
Thus this invention provides a system for rapidly and accurately
measuring the air-fuel ratio of combustible mixtures supplied to
combustion mechanisms. The system is useful not only for analytical
purposes but also as an element in a control loop for automatically
controlling air or fuel supplies to produce or maintain desired
air-fuel ratios.
* * * * *