GB1562623A - Device for monitoring the composition of the gaseous emission of a combustion process - Google Patents

Description

(54) A DEVICE FOR MONITORING THE COMPOSITION OF THE GASEOUS EMISSION OF A COMBUSTION PROCESS (71) We, LUCAS INDUSTRIES LIMITED, a British Company, of Great King Street, Birmingham, B19 2XF, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a device for monitoring the composition of the gaseous exhaust emission of a combustion process and, in particular, of an internal combustion engine.

A device, according to one aspect of the invention, includes an insulating substrate which, at the elevated operating temperature of the device, is inert to the gaseous exhaust emission being monitored, a metal oxide exhaust emission sensing element adhering to the insulating substrate so as to be exposed, in use, to the exhaust emission being monitored, the electrical conductivity of said metal oxide varying at said elevated operating temperature with the concentration of oxidising or reducing constituents in the exhaust emission, means for heating the sensing element to said elevated operating temperature, and a temperature sensor located adjacent said exhaust emission sensing element so as to be heated, in use, by said means to substantially the same temperature as said exhaust emission sensing element whereby the output from said sensor can be used to control said heating means such that said exhaust emission sensing element is maintained at said elevated operating temperature.

Preferably, the heating means includes a resistive layer adhering to the substrate.

Preferably, the temperature sensor also includes a resistive layer adhering to the substrate, the resistance of said layer varying in known manner with temperature.

More preferably, the sensing element and said temperature sensor adhere to one surface of the substrate and said heating means adheres to an opposite surface of the substrate.

Most preferably, said heating means is located on said opposite surface of the substrate so as to be aligned with said sensing element and said sensor.

Preferably, the substrate, the sensor, the sensing element and the heating means are supported by a body which includes a shroud extending around the sensing element out of contact therewith so as to prevent the exhaust emission flowing directly over the sensing element in use.

Preferably, the metal oxide of the sensing element is titanium dioxide or gallia oxide.

Alternatively the oxide is zinc oxide or nickel oxide.

Preferably, the substrate includes a sintered ceramic tile to which the metal oxide sensing element is applied by printing techniques and is then fired.

Preferably, each of said heating means and said temperature sensor is also applied to the tile by printing techniques and is then fired.

In a further aspect, the invention resides in a system for monitoring the composition of the exhaust emission of an internal combustion engine, comprising a device as described in the preceding paragraphs, and means for producing an output signal the value of which is dependent on the conductivity of said metal oxide.

In yet a further aspect of the invention resides in a system for controlling the supply of fuel to an internal combustion engine, comprising an exhaust emission monitoring system as described in the preceding paragraph, means for supplying a fuel/air mixture to the engine and means for controlling operation of said supply means in accordance with the value of said output signal so that the composition of said fuel/air mixture supplied to the engine depends on the com position of the exhaust emission produced by the engine.

In the accompanying drawings, Figure 1 is a sectional view of a device according to one example of the invention for monitoring the composition of the exhaust emission of an internal combustion engine, Figure 2 is a plan view to an enlarged scale showing in detail the arrangement of the exhaust emission sensing element and the temperature sensor of the device shown in Figure 1, and, Figures 3 and 4 are sectional views illustrating two alternative forms respectively of the heater of the device shown in Figure 1.

Referring to the drawings, the device includes an elongated, sintered ceramic tile which at one end supports a metal oxide, exhaust emission sensing element 12, a temperature sensor 13 and a heater 14. The sensing element 12 and sensor 13 are printed adjacent one another on one major surface of the tile 11, whereas the heater 14 is printed on the opposite major surface of the tile so as to aligned with the element 12 and sensor 13.

Preferably, the ceramic material of the tile 11 is alumina and the tile is arranged to have a low glass content in order to ensure a high mechanical strength and provide a high electrical resistivity. For adequate resistivity a minimum alumina content of 99% is preferred and in one practical embodiment, a satisfactory tile was obtained from a 0.025 inch thick sintered, alumina plate which was supplied by Bush Beech Engineering Limited of Cheshire, England and which had a 99.5% alumina content, the plate being cut to a rectangle 1.89 inch in length and 0.265 inch in width to produce the tile 11.

The sensing element 12 is composed of a sintered mixture of titanium dioxide with lmol% of tantalum oxide and 1 atomic % of platinum. To produce this mixture, the components are heated together in air at 11000C for two hours, with the platinum conveniently being introduced into the starting material as platinous chloride. The sin tered product is then ground to a powder which is classified by settling in water or any other suitable non-reactive fluid so as to remove particles of size in excess of 10 mic rons. The classified powder, which of course consists of particles of 10 microns and less in size, is then mixed with an organic carrier liquid, whereafter the resultant suspension is applied to the tile by conventional thick film screen printing techniques.Any con ventional carrier liquid can be employed as the suspending phase and, for example, the carrier liquid supplied by Englehard Sales Limited as Medium 4/730 has been found to produce satisfactory results. It is, however, preferable to ensure that the suspension contains between 30 and 35 volume percent of the carrier liquid, which is considerably higher than the range of 15 to 25 volume percent normally employed in thick film printing techniques. By the same token, it is to be appreciated that using conventional thick film printing techniques, the particulate sensing material normally has a particle size of between 0.1 and 2 microns, while as previously stated the present example employs a particle size of up to 10 microns.As will become apparent below, these departures from conventional procedure result in the production of a sensing element 12 having advantageous properties.

After the powder suspension has been printed on the tile 11, the resultant layer is dried at 1200C for half to one hour and is then fired at 12000C to sinter the mixture into the required sensing element 12. The sintering operation is found to produce a strong bond between the element 12 and the tile 11 without the necessity for introducing a glass to improve the adhesion. Thus, using the mixture described above, it is found that the element 12 remains strongly adhered to the tile 11 even at the high temperatures (of the order of 900"C) which the element experiences in use.Sintering at the relatively high temperature of 1200"C is also necessary in order that the sensing element will show good stability at its operating temperature of 900"C. Moreover the resultant sintered film is porous which is found to result in an improved response time for the device since the surface area exposed to the gases being sensed is relatively greater compared to the volume of material. In addition, the porosity of the element 12 enables improved electrical connections to be made to the element.

The element 12 conveniently has a resistance value in air of 1M ohms and preferably is shaped so as to have a low aspect ratio (length measured between conductive connectors; width measured parallel to connectors). Thus, in the preferred example shown, the element 12 is a rectangular configuration.

The sensor 13 is a platinum resistance thermometer which is printed on the tile 11 in a similar manner to that described for the element 12, although in this use the printing medium is a platinum ink, in which the platinum particles preferably have a particle size no greater than 2 microns so as to enable the printing of a pattern with an accurate fire line width. The resultant layer is then dried at 1200C and thereafter fired for 1 hour at 12000C to 13000C. The firing operation is a longer and hotter process than is normal for platinum thick films, but this is desirable to promote ageing processes in the film so that the resistance of the film will be stable in service.The resultant sensor 13 is arranged to have a resistance value in air at 20"C of the order of 10 ohms and preferably has a high aspect ratio, the latter being achieved in the preferred example by arranging the sensor 13 in the form of a narrow strip having the zig-zag configuration shown in Figure 2 and provided with an enlarged terminal portion 1 3a at each end.

After firing, the sensor 13 is preferably protected by overprinting at this stage of a glass passivation layer which is used to minimise the effect of gas composition on the resistance and stability of the platinum film. A suitable glass is available from E.M.C.A. as Type 1340 dielectric glass.

This glass is dried in air and then fired in air.

conventionally at 12000C.

When formation of the element 12 and sensor 13 is complete, first and second pairs of conductive areas 15, 16 are printed on the tile 11 to provide the necessary electrical connections to the element 12 and sensor 13 respectively. Each conductive area extends along the length of said one surface of the tile 11, with the areas 16 overlapping the terminal portions 1 3a respectively while the areas 15 extend over opposite edges respectively of the rectangular element 12. Conveniently, each area 15, 16 is formed by applying to the tile 11 an ink composed of gold and platinum together with a high melting point glass to aid bonding.The ink is applied by conventional thick film printing techniques, whereafter the printed layers are dried in air at 1200C for 30 minutes and then fired at 1150"C to produce the required areas. Conveniently each of the areas 15, 16 is arranged to have a thickness between 10 and 25 microns, more preferably 15 microns, and a resistance of about 3.5 ohms. This resistance value is sufficiently small to allow the areas 15 to provide satisfactory electrical connections to the element 12, since of course the element 12 is a high resistance device. The resistance of the sensor 13 is, however, normally of the order of 10 ohms and hence requires very low resistivity connections.Thus, after the layers 16 have been fired they are overprinted with gold, conveniently supplied by E.M.C.A. as Type 213U, the resultant gold layers subsequently being dried and fired at 800"C to provide connections to the sensor 13 having a resistance of about 0.2 ohms.

In a modified arrangement for providing the electrical connections to the sensor 13, the terminal portions 13a of the sensor are arranged so as to extend along the length of the tile 11. Gold layers are then printed directly onto the terminal portions 13a and are fired as described above to provide the necessary low resistivity connections, the gold again being that supplied by E.M.C.A.

as Type 213U. In this modification, the electrical connections to the element 12 are produced in the same way as described in the preceding paragraph.

In a further modification, the areas 15, 16 are produced by printing directly onto the tile 11 a gold-based, reactive bond, thick film ink of the kind supplied by E.M.C.A. as Type 3264, the printed layers being subsequently dried and then fired at 10000C. The resistance of the resultant areas 15, 16 is sufficiently low to provide the required electrical connections to both the element 12 and the sensor 13.

Referring now to Figure 3, the heater 14 is produced by printing over the whole of said opposite surface of the tile 11 either a platinum composition, of the type supplied by Englehard Ltd., as No. 6082, or the platinum-gold composition supplied by E.M.C.A. as Type 180. The printed layer is then dried and fired at 12000C in the case of the platinum-gold composition or at 1 1500C in the case of the platinum composition to produce the sintered film indicated at 17. A pair of conductive areas 18 (see also Figure 1) are then printed over the film 17, with the end portion of the film adjacent the element 12 and sensor 13 being exposed so as to define the heater 14. The conductive areas 18 provide the required electrical connections to the heater 14 and conveniently are produced from the unfluxed, pure gold printing composition supplyed by E.M.C.A.

as Type 213U. After application of the printing composition, the resultant printed layers are fired at 800"C to produce the required connectors, which conveniently have a resistance of about 150m ohms as compared with a resistance of 1 to 3 ohms for the heater 14.

Referring to Figure 4, in a modification of the arrangement described in the preceding paragraph, the heater 14 is produced from the same platinum or platinum/gold printing composition used previously, but now the printing composition is only applied to part of said other surface of the tile 11. Thus the printing composition is applied to the area of said other surface which is to define the heater 14 together with a short extension 14a for enabling electrical connections to be made to the heater 14. The electrical connections are produced by printing the conductive areas 18 directly onto said other surface of the tile 11 and over the extension 14a. The material used to produce the areas 18 is now the reactive gold printing composition supplied by E.M.C.A. as Type 3264 and, after printing, the tile is fired at 1000"C to produce the required areas 18.Again, the areas 18 are arranged so as to have a resistance of about 150m ohm as compared with a resistance of 1 to 3 ohms for the heater 14.

In both the arrangements shown in Figure 3 and the modification shown in Figure 4, the heater 14 is preferably coated with a glass layer (not shown) to improve the long term stability of the heater. Where the heater 14 is produced from the platinum composition supplied by Englehard as Type 6H82, the glass layer is preferably produced by coating the heater 14 with a glass composition supplied by Englehard as Type 33/360 and then firing this composition at 975"C. Alternatively where the heater 14 is formed from the platinum/gold composed tion supplied by E.M.C.A. as Type 180, the glass layer is preferably produced by coating the heater 14 with the glass composition supplied by Dupont as Type 8299 and then firing this composition at 1000"C. A further glass layer 22 (Figure 2) is conveniently also provided over the sensor 13.

In producing the device described above, the appropriate patterns are first printed and fired on the tile 11, whereafter pairs of gold wires 23, 24, 25 are joined to the pairs of conductive areas 15, 16, 18 respectively by thermo-compression bonding. Supporting the wires 23, 24, 25 is a cylindrical ceramic body 26 which at one end is formed with an axially extending groove 27 in which the tile 11 is secured. At its other end, the body 26 is joined to a flanged, stainless steel tube 28, both joints to the body 26 being formed by means of a ceramic cement such as that supplied by Carlton Brown and Partners Limited Autostic or by a low melting point glass. Closing the free end of the tube 28 is a glass seal 29 carrying six metal tubes 31 in which the wires 23, 24, 25 respectively are secured by soldering or crimping.

Extending around the tile 11 and body 26 is a heat resistant, hollow cylindrical body 32 which is in screw-threaded engagement with a flanged nut 33 so that the flange on the tube 28 is trapped between the nut 33 and the body 32. Conveniently, the body 32 is filled with an impervious ceramic material, conveniently a siliceous alumina cement, so as to enclose the major portion of the tile 11 but leave the element 12, sensor 13 and heater 14 exposed. Moreover, the open end of the body 32 is preferably closed by an apertured end cap so that the element 12 is shrouded so as to prevent exhaust emission flowing, in use, directly over the element.

When the device described above is used to monitor the composition of exhaust emission from, for example, an internal combustion engine, current is supplied by way of the wires 25 and the conductive areas 18 to the heater 14 so that the element 12 is heated to its required operating temperature, normally 900"C. At this temperature, the electrical conductivity of the element 12 varies in accordance with the amount of oxidising or reducing constituents in the exhaust emission and hence a measure of the emission composition can be obtained by connecting the wires 23 to a voltage source and then measuring the current flowing through the element 12. In order to obtain accurate sensing, however, it is necessary to ensure that the element 12 is maintained at a substantially constant temperature.This is readily achieved in the example described since the temperature sensor 13 is positioned adjacent the element 12 and hence, in use, will be heated by the heater 14 to substantially the same temperature as the element 12. Thus, by monitoring the resistance of the sensor 13, which is of course temperature dependent, the temperature of the element 12 can be closely followed and, where necessary, the current supplied to the heater 14 varied to retain the element 12 at its required operating temperature.

The device described above conveniently forms part of a system for controlling the supply of fuel to an internal combustion engine with which the device is associated.

In such a system, the output from the element 12 is fed to means for controlling operation of injectors or carburettors for supplying a fuel/air mixture to the engine.

The arrangment is then such that operation of the injectors or carburettors is controlled in accordance with the value of this output signal so that the composition of the fuel/air mixture supplied to the engine can be controlled in accordance with the composition of the exhaust emission produced by the engine.

As an alternative to the example described above, gallia (Ga203) can be used instead of titania as the metal oxide of the detecting element 12, in which case the sintering of the element is again preferably arranged to produce a porous film since operation of a gallia sensor involves surface reactions which are enhanced by a large surface area. Using a gallia sensor, however, it is, necessary to print a layer of a devitrifying glass onto the tile 11 below the metal oxide layer so as to ensure bonding of the element 12 to the tile 11. Further metal oxides which could be used instead of the titania of the above example include zinc oxide and nickel oxide.

WHAT WE CLAIM IS: 1. A device for monitoring the composition of the gaseous exhaust emission of a combustion process, the device including an insulating substrate which, at the elevated operating temperature of the device, is inert to the gaseous exhaust emission being monitored, a metal oxide exhaust emission sensing element adhering to the insulating substrate so as to be exposed, in use, to the exhaust emission being monitored, the electrical conductivity of said metal oxide varying at said elevated operating temperature with the concentration of oxidising or reduc

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. the heater 14 is preferably coated with a glass layer (not shown) to improve the long term stability of the heater. Where the heater 14 is produced from the platinum composition supplied by Englehard as Type 6H82, the glass layer is preferably produced by coating the heater 14 with a glass composition supplied by Englehard as Type 33/360 and then firing this composition at 975"C. Alternatively where the heater 14 is formed from the platinum/gold composed tion supplied by E.M.C.A. as Type 180, the glass layer is preferably produced by coating the heater 14 with the glass composition supplied by Dupont as Type 8299 and then firing this composition at 1000"C. A further glass layer 22 (Figure 2) is conveniently also provided over the sensor 13. In producing the device described above, the appropriate patterns are first printed and fired on the tile 11, whereafter pairs of gold wires 23, 24, 25 are joined to the pairs of conductive areas 15, 16, 18 respectively by thermo-compression bonding. Supporting the wires 23, 24, 25 is a cylindrical ceramic body 26 which at one end is formed with an axially extending groove 27 in which the tile 11 is secured. At its other end, the body 26 is joined to a flanged, stainless steel tube 28, both joints to the body 26 being formed by means of a ceramic cement such as that supplied by Carlton Brown and Partners Limited Autostic or by a low melting point glass. Closing the free end of the tube 28 is a glass seal 29 carrying six metal tubes 31 in which the wires 23, 24, 25 respectively are secured by soldering or crimping. Extending around the tile 11 and body 26 is a heat resistant, hollow cylindrical body 32 which is in screw-threaded engagement with a flanged nut 33 so that the flange on the tube 28 is trapped between the nut 33 and the body 32. Conveniently, the body 32 is filled with an impervious ceramic material, conveniently a siliceous alumina cement, so as to enclose the major portion of the tile 11 but leave the element 12, sensor 13 and heater 14 exposed. Moreover, the open end of the body 32 is preferably closed by an apertured end cap so that the element 12 is shrouded so as to prevent exhaust emission flowing, in use, directly over the element. When the device described above is used to monitor the composition of exhaust emission from, for example, an internal combustion engine, current is supplied by way of the wires 25 and the conductive areas 18 to the heater 14 so that the element 12 is heated to its required operating temperature, normally 900"C. At this temperature, the electrical conductivity of the element 12 varies in accordance with the amount of oxidising or reducing constituents in the exhaust emission and hence a measure of the emission composition can be obtained by connecting the wires 23 to a voltage source and then measuring the current flowing through the element 12. In order to obtain accurate sensing, however, it is necessary to ensure that the element 12 is maintained at a substantially constant temperature.This is readily achieved in the example described since the temperature sensor 13 is positioned adjacent the element 12 and hence, in use, will be heated by the heater 14 to substantially the same temperature as the element 12. Thus, by monitoring the resistance of the sensor 13, which is of course temperature dependent, the temperature of the element 12 can be closely followed and, where necessary, the current supplied to the heater 14 varied to retain the element 12 at its required operating temperature. The device described above conveniently forms part of a system for controlling the supply of fuel to an internal combustion engine with which the device is associated. In such a system, the output from the element 12 is fed to means for controlling operation of injectors or carburettors for supplying a fuel/air mixture to the engine. The arrangment is then such that operation of the injectors or carburettors is controlled in accordance with the value of this output signal so that the composition of the fuel/air mixture supplied to the engine can be controlled in accordance with the composition of the exhaust emission produced by the engine. As an alternative to the example described above, gallia (Ga203) can be used instead of titania as the metal oxide of the detecting element 12, in which case the sintering of the element is again preferably arranged to produce a porous film since operation of a gallia sensor involves surface reactions which are enhanced by a large surface area. Using a gallia sensor, however, it is, necessary to print a layer of a devitrifying glass onto the tile 11 below the metal oxide layer so as to ensure bonding of the element 12 to the tile 11. Further metal oxides which could be used instead of the titania of the above example include zinc oxide and nickel oxide. WHAT WE CLAIM IS:

1. A device for monitoring the composition of the gaseous exhaust emission of a combustion process, the device including an insulating substrate which, at the elevated operating temperature of the device, is inert to the gaseous exhaust emission being monitored, a metal oxide exhaust emission sensing element adhering to the insulating substrate so as to be exposed, in use, to the exhaust emission being monitored, the electrical conductivity of said metal oxide varying at said elevated operating temperature with the concentration of oxidising or reduc

ing constituents in the exhaust emission, means for heating the sensing element to said elevated operating temperature, and a temperature sensor located adjacent said exhaust emission sensing element so as to be heated, in use, by said means to substantially the same temperature as said exhaust emission sensing element whereby the output from said sensor can be used to control said heating means such that said exhaust emission sensing element is maintained at said elevated operating temperature.

2. A device as claimed in claim 1 wherein the heating means includes a resistive layer adhering to the substrate.

3. A device as claimed in claim 2 wherein the temperature sensor also includes a resistive layer adhering to the substrate, the resistance of said layer varying in known manner with temperature.

4. A device as claimed in claim 3 wherein the sensing element and said temperature sensor adhere to one surface of the substrate and said heating means adheres to an opposite surface of the substrate.

5. A device as claimed in claim 4 wherein said heating means is located on said opposite surface of the substrate so as to be aligned with said sensing element and said sensor.

6. A device as claimed in any one of the preceding claims wherein the substrate, the sensor, the sensing element and the heating means are supported by a body which includes a shroud extending around the sensing element out of contact therewith so as to prevent the exhaust emission flowing directly over the sensing element in use.

7. A device as claimed in any one of claims 1 to 6 wherein the metal oxide of the sensing element is titanium dioxide or gallia oxide.

8. A device as claimed in any one of claims 1 to 6 wherein the metal oxide of the sensing element is zinc oxide or nickel oxide.

9. A device as claimed in any one of the preceding claims wherein the substrate includes a sintered ceramic tile to which the metal oxide sensing element is applied by printing techniques and is then fired.

10. A device as claimed in any one of the preceding claims wherein each of said heating means and said temperature sensor is also applied to the tile by printing techniques and is then fired.

11. A device for monitoring the composition of the gaseous exhaust emission of a combustion process comprising the combination and arrangement of parts substantially as hereinbefore described with reference to and as shown in Figures 1 and 2 or Figure 3 or Figure 4 of the accompanying drawings.

12. A system for monitoring the composition of the exhaust emission of an internal combustion engine, comprising a device as claimed in any one of the preceding claims, and means for producing an output signal the value of which is dependent on the conductivity of said metal oxide.

13. A system for controlling the supply of fuel to an internal combustion engine, comprising an exhaust emission monitoring system as claimed in claim 12, means for supplying a fuel/air mixture to the engine and means for controlling operation of said supply means in accordance with the value of said output signal so that the composition of said fuel/air mixture supplied to the engine depends on the composition of the exhaust emission produced by the engine.