US3893230A

US3893230A - Method of manufacture of an exhaust gas sensor for an air-fuel ratio sensing system

Info

Publication number: US3893230A
Application number: US391424A
Authority: US
Inventors: Henry L Stadler; Tseng-Ying Tien; Michael J Esper; Donald J Romine
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1971-11-15
Filing date: 1973-08-23
Publication date: 1975-07-08
Anticipated expiration: 1992-07-08

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.

Description

United States Patent 1191 Stadler et al.

[ METHOD OF MANUFACTURE OF AN EXHAUST GAS SENSOR FOR AN AIR-FUEL RATIO SENSING SYSTEM [75] Inventors: Henry L. Stadler; Tseng-Ying Tien,

both of Ann Arbor; Michael J. Esper, Detroit; Donald J. Romine, Southfield, all of Mich.

[731 Assignee: Ford Motor Company, Dearborn,

Mich.

[221 Filed: Aug. 23, 1973 [21] Appl. No.: 391,424

Related US. Application Data [63] Continuation of Ser. No, 198,515, Nov 15, 1971,

abandoned.

[52] US. Cl. 29/592; 23/232 E, 23/254 E; 29/625; 156/89 {51] Int. Cl. GOln 29/02 [58] Field of Search 23/232 E, 254 E, 255 E;

204/195 P, 195 M, 195 S; 73/27 R, 26; 338/34, 249, 254, 255, 256, 314; 29/592, 610, 612, 613, 614, 625, 627; 156/89, 228,

[56] References Cited UNITED STATES PATENTS 2,389,420 11/1945 Deyrup 29/625 UX 1 July 8,1975

2,427,212 3/1948 Schottland 29/625 UX 2,470,509 5/1949 Marini A 29/625 UX 3.040113 6/1962 Byer et a1, 29/625 UX 3,189,974 (3/1965 Fabricus 29/625 UX 3,189,978 6/1965 Stetson 29/625 3,518,756 7/1970 Bennett et al. 29/625 3,604,082 9/1971 McBrayer et al. 65/59 UX 3,695,848 10/1972 Taguchi 1 23/254 E 3,751,968 8/1973 LOh el al .1 23/254 E X Primary Examiner-C. W. Lanham Assistant Examinerloseph A. Walkowski Attorney, Agent, or Firm-Robert A. Benziger; Keith L. Zerschling [57] 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.

6 Claims, 3 Drawing Figures P'A'TENTEBJuL e ms 3 893; 230

F I cs .1

i L/e? LL 1 l I L POWER Z6 SUPPLY RESISTANCE SENSOR METHOD OF l\I.\NUFA(TTL'RIi OF AN EXHAUST GAS SENSOR FOR AN AIR-FUEL RATIO SENSING SYSTEM This is a continuation of application Ser. 198,515. filed Nov. I5. I971, now abandoned.

SUMMARY OI" 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 airfuel 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 airfuel 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 oxidationreduction 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 combusoxides such as titanium dioxide. vanadium oxide. chromium oxide. manganese oxide. iron oxide. nickel oxide. cobalt oxide. and rare earth metal oxides such as cc rium 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.

Scnsing 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 unvapor' ized fuel. The system of the invention is useful particw larly 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 -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 600-900C.. produces adequate indications of the air-fuel ratio supplied to an engine despite the fact that temperature var iations change the resistance between the electrodes. Temperatures below 600C tend to coat the member with soot and other particulate impurities while tem peratures above 900C 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. I is a schematic illustration of a reciprocating internal combustion engine showing the installation in the exhaust pipe of a sensing member ofthis invention.

PK]. 2 is a schematic plan view of a disc-shaped sens ing member of this invention showing the electrical connections thereto.

FIG. 3 is a sectional elevation of the diseshaped sensing member of FIG. 2 showing the disposition of the electrodes. an electrical resistance heating wire. and a thermocouple in a sandwich construction.

DETAlLED DESCRIPTION Referring to FIG. 1. a reciprocating internal combus tion engine includes an intake manifold [2 for deliv ering an air-fuel mixture to the engine combustion chambers (not shown) and an exhaust manifold H for removing the combustion products from the combus tion chambers. A carburetor 16 is attached to the intake manifold and an air cleaner I8 is attached to the air inlet of carburetor l6. Carburetor 16 receives fuel from a fuel source (not shown). produces an air-fuel mixture and supplies the airfuel mixture to intake manifold 12.

Exhaust manifold I4 is connected to an exhaust pipe 20. Threaded into the wall ofexhuast 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 ofelectrieal resistance wire 36 is sandwiched between plates 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 shown in FIG. 2. The

entire sandwich is fired into a unitary structure by con vcntional 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 ther mocouple 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 al loyed 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 [0,000 psi. After pressing. the assembly is fired at about 23002500F. 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 C. When the engine is supplied with an air-fuel mixture of about 13: l. the resistance across the electrodes is about 2000 ohms. (hanging the airt'ucl ratio to l4:l without changing any other engine parameters increases the rcsistance to about 7000 ohms. An air tuel ratio of [5:1 produces a resistance of 20.000 ohms. Other embodiments produce resistances ranging from I00 ohms at alt air fuel ratio ot l 1:] to 500.000 ohms at a ratio of 16: I.

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 signifi' cantly the shape of the curve. Resistance changes rap idly 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 airfuel ratios.

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

tiring 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 of theoretical to about 959? 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 2300F to about 2S00F for a period of time less than about two hours.

6. The process of claim I 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 [0,000

psi.

Claims

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.

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*F to about 2500*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.