GB2135462A

GB2135462A - Monitoring gas composition

Info

Publication number: GB2135462A
Application number: GB08404656A
Authority: GB
Inventors: John Maxwell Dyer
Original assignee: AUTOTECH DEVELOPMENTS Ltd
Current assignee: AUTOTECH DEVELOPMENTS Ltd
Priority date: 1983-02-22
Filing date: 1984-02-22
Publication date: 1984-08-30
Also published as: GB8404656D0

Abstract

Gaseous substances in a gaseous stream, particularly carbon monoxide and hydrocarbons in the exhaust emission of an internal combustion engine are monitored using one or more electronic sensor devices (34) within an enclosure (30) which receives gas from the gaseous stream via an opening (35). Electrical properties of the sensor devices may be monitored with electronic circuitry which controls actuation of rows of light- emitting diodes. The enclosure is defined by a tube mounted on the top of a pipe 20 through which the gaseous stream flows. A Pitot tube 23 is used to divert part of the gaseous stream into and through the enclosure. <IMAGE>

Description

SPECIFICATION Monitoring gas content This invention relates to a method and apparatus for monitoring the content of one or more gaseous substances in a gaseous stream.

The invention is more particularly, although not exclusively, concerned with the measurement of the proportions of gases such as carbon monoxide and hydrocarbons in the exhaust emissions of internal combustion engines.

Widespread concern as to atmospheric pollution caused by exhaust emissions of internal combustion engines has led to the introduction of recommended or obligatory limits with regard to the content of noxious gases, particularly carbon monoxide and hydrocarbons. Compliance with these limits generally involves careful selection of engine design and 'tuning' or adjustment of operational parameters such as fuel mixture. It is also known to effect engine tuning with reference to exhaust gas content with a view to optimising performance in terms of power output and/or economy.

Known procedures for monitoring exhaust gas content are based on non-dispersive infrared analysis techniques and therefore involve the use of expensive and complicated equipment.

An object of the present invention is to enable the content of exhaust gases to be monitored reliably using relatively simple and inexpensive equipment.

According to one aspect of the present invention therefore there is provided a method of monitoring the content of one or more gaseous substances in a gaseous stream, which method involves the use of an electronic device which is adapted to undergo a change in an electrical property thereof when exposed to at least one said substance and which is disposed within an enclosure having an opening thereto, and wherein said opening is exposed to said gaseous stream so that said enclosure fills with gas therefrom, and the said electrical property of the said device is monitored.

With this arrangement due to the use of the said gas-sensitive electronic device, the method of the invention can be performed without necessitating complex and expensive apparatus yet at the same time reliability can be readily achieved in so far as predictable conditions with regard to the exposure of the device to the gas can be achieved due to the effect of the enclosure. In the latter respect, it is particularly preferred that the arrangement of the enclosure in relation to the gas stream should be such that the enclosure becomes filled with a static or nearly static or predictably slow moving mass of gas.

Thus, the said enclosure may be closed or substantially closed except for the said opening and/or the opening may be arranged peripherally at an angle to the direction of movement of the gas stream and/or the enclosure may have applied thereto a gas-flow restricting element or structure such as a sintered metal filter plate.

If desired there may be two or more separately monitored said electronic devices disposed in the same enclosure or alternatively in different respective said enclosures and adapted to be responsive (mainly or wholly) to two different said gaseous substances.

With regard to the monitoring of the changeable electrical property of the or each said device this may be effected in any suitable manner as desired. Thus monitoring may be effected in a relatively simple qualitative manner whereby for example a visual or audible indicator is actuated in the event that a predetermined limit is exceeded; or, alternatively a more precise measurement system producing a quantitative output in the form of a digital or analogue meter reading or the like may be involved.

It is visualised that the method of the invention will find particular application in a garage context for monitoring the content of noxious gaseous substances such as carbon monoxide and hydrocarbons in the exhaust emissions of road vehicle internal combustion engines, and accordingly, the arrangement may be such that the or each said enclosed electronic device may be adapted to be removably applied to the tail end of a vehicle exhaust pipe. It is however to be understood that the invention is not intended to be restricted to this field of application and the method thereof may be applied to the monitoring of any suitable gaseous substances in any suitable context whether on an occasional or continual basis and whether by permanent or only temporary association of the or each said electronic device with the gas stream to be monitored.

According to a second aspect of the present invention there is provided apparatus for use in performing the above described method comprising an enclosure having an opening thereto for exposure to a gas stream, an electronic device within the enclosure which is adapted to undergo a change in an electrical property thereof when exposed to at least one predetermined gaseous substance in said stream, and electrical monitoring equipment sensitive to changes in the said property of the said device and operable to produce an output in response thereto. The apparatus may incorporate subsidiary features in correspondence with the above described subsidiary method features.

Conveniently the said enclosure may comprise a tube, one end of which has said opening thereto, said tube being fixed externally to one side of a pipe with said opening in communication with a hole in the pipe wall, whereby gas from said gas stream flowing along said pipe can enter said enclosure through said opening.

Where the apparatus is to be used with a vehicle exhaust system the said pipe may be dimensioned to fit tightly yet removably over or within the tail end of the exhaust, or alternatively an adaptor in the form of a generally conical sleeve formed for example from a heat resistant rubber may be provided on one end of the pipe to enable different sizes of exhaust tails to be accommodated.

The invention will now be described further by way of example only and with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic sectional representation of equipment forming part of one form of apparatus according to the present invention; Figs. 2 to 5 and 7 are circuit diagrams of alternative embodiments of the apparatus of the invention; Fig. 6 is a view similar to Fig. 1 of an alternative version of the equipment.

With reference to Fig. 1, apparatus for use in monitoring the content of carbon monoxide and hydrocarbons in motor vehicle exhaust emissions comprises a short cylindrical open-ended steel pipe 1 which is fixed at one end within the smaller end of a frusto-conical heat-resistant rubber sleeve 2.

There are holes 3 at two diametrically opposed positions in the pipe side wall and two narrow open-ended cylindrical steel tubes 4 are welded at ends thereof to the pipe 1 around these holes 3. The tubes 4 extend at common angles to the pipe axis inclined away from the rubber sleeve 2.

Each tube 4 contains a drum-shaped gassensitive device 5 (yet to be described) which fits tightly in the central region of the tube 4 and has electrical terminals 6 at the side thereof facing away from the pipe 1. A sintered metal disc 7 is fixed in each tube 4 between the device 5 and the pipe 1. The ends of the tubes 4 remote from the pipe 1 are closed with plastics caps 8 and electrical leads 9 joined to the terminals 6 pass through holes in the caps 8 and are connected to monitoring equipment yet to be described.

Each device 5 is a known, proprietary semiconductor (tin oxide) gas sensor of the kind comprising a ceramic tube a few millimetres long inside which is wound a low voltage (5V d.c.) heater filament. The outer surface of the ceramic tube is coated with sintered tin oxide containing catalytic dopants and there are two annular electrodes at opposite ends of the ceramic tube in good contact with the coating. The ceramic tube is mounted within a plastics capsule having the said terminals 6 at one side and being open at the opposite side via a double stainless steel gauze.

In use, the filament is powered to heat the ceramic tube and the electrical resistance between the electrodes is monitored. In fixed conditions of temperature and humidity, an equilibrium state is reached in which the surface conductance between the electrodes is defined by the concentration of adsorbed oxygen. Any contaminating gas will also be adsorbed and may react with the oxygen thereby changing the surface conductance.

The sensor can be rendered selectively sensitive to different gases by appropriate selection of the catalytic dopants and, in the present case, one device 5 may be more sensitive to carbon monoxide and the other device 5 may be more sensitive to hydrocarbons.

The conductance of each device 5 may be monitored using an evaluation circuit comprising a resistive bridge circuit and a comparator i.c. 10 as shown in Fig. 2 or alternatively comprising an operational amplifier 11 with a constant reference voltage as shown in Fig. 3. Substantially linear outputs can be obtained which are proportional to measured gas concentration for the two devices and these can be used to produce suitable visual display which conveniently may be arranged side by side and respectively identified carbon -monoxide (CO) and hydrocarbon (HC).

As shown in Fig. 4, the two meter displays may comprise rows of LEDs, the evaluated outputs of the devices being processed by a suitable analogue to digital converter circuit operable to power the LEDs. For example, the voltage swing derived from each gas sensor device may be applied to the input of a Teledyne CMOS 8700 series device 12 and this may be configured to drive a matrix of LEDs 13 through binary to one of four decoders 14, thereby providing the possibility of powering up to 64 discrete diodes in a string.

Thus a single illuminated diode may represent a value of one sixty-fourth of the scale length which might for example be 0.25% CO or 10 to 400 ppm HC. The resolution of the system with this method of read-out is (within the device linearity), 0.075% CO per step or 6.094 ppm HC per step.

With the arrangement of Fig. 5 the detected voltage swing is derived in the "constant voltage" mode, by employing a "Norton" amplifier e.g. an LM 3900 integrated circuit 1 5. The amplifier output is pre-attenuated and fed to the drive circuitry of a microprocessor controlled digital instrument 1 6 which provides the following display modes/functions: 1. Digital read-out to 9999; 2. Bar graph (trend bar); 3. Scale multiplier; 4. Function indicator i.e. CO, HC, RPM, etc.

The digital instrument is combined with those circuits necessary to measure carbon monoxide (CO) to a maximum of 10% CO, and HC to a maximum to 800 ppm. Both CO and HC measurements are arranged with alternative scale multiplication factors as appropriate. In order to provide an indication of engine speed (RPM), a tachometer feature may also be incorporated to operate through one or other instrument by switch selection.

In each case it will be appreciated that initial circuit calibration is required and this may be achieved with the devices exposed to a static mass or controlled flow of gas which is of predetermined known composition or which is analysed during calibration with infra-red or other apparatus. Adjustment for calibration purposes can also be made by selection of the sintered disc filters.

In use, the apparatus shown in Fig. 1 is applied to the tail end of a motor vehicle exhaust pipe 17, by pushing the conical adaptor over the exhaust pipe, and the devices are linked by the leads to the monitoring equipment comprising the circuitry and visual displays as discussed with reference to Figs. 2 to 5 which conveniently may be housed in a control unit for example of a "desk top" format. The vehicle engine is then run until the meter indications stabilize and the content of carbon monoxide and hydrocarbons can be read off in qualitative or quantitative terms as appropriate. It will be appreciated that provision can also be made for production of an output for operation of a printer, for feed to a computer or for any other suitable purpose. After gas content has been tested the adaptor is removed from the tail pipe and can be re-used with a further vehicle.

Fig. 7 shows a modified version of the equipment of Fig. 1.

The modified equipment comprises a short cylindrical open-ended steel pipe 20 which is fixed at one end around the smaller end of a stepped frusto-conical heat-resistant rubber sleeve 21.

A Pitot tube 22 is fixed within the pipe 20, such tube having a straight portion 23 at one end which extends coaxially relative to the pipe 20 and terminates in a restricted interchangeable calibrated inlet jet 24 facing towards the rubber sleeve 21. The opposite end portion 25 is turned upwardly through 900 and is passed through and is secured in position relative to a side hole 26 in the top of the pipe 20. The top end 27 of the tube 22 is sealed off and a discharge outlet 28 say 2 mm in diameter is provided in the side of the tube slightly below the top end 27 externally of the pipe 20. A short, wide tube 29 of oval crosssection is seated on the outside of the pipe 20 around the top end 27 of the tube 22, such tube 29 being arranged with its long diameter parallel to the axis of the pipe 20 and being sealed relative to the pipe 20 around its lower periphery.

The tube 29 is divided into upper and lower compartments 30, 31 by a partition 32. The upper compartment 30 is closed by a transverse top wall 33 and contains two (or more) sensors 34 of like kind to the sensors 5 described with reference to Fig. 1. These sensors 34 have terminals which are connected by leads (not shown) passing through an outlet 37 in the lower compartment 31 to monitoring equipment as described above with reference to Figs. 2 to 5 or as described hereinafter with reference to Fig. 6.

The lower compartment 31 is in communication with the upper.compartment via a tube 35 arranged coaxially around the top end 27 of the tube 22 and fixed to the pipe 20. This tube 35 is open at its top end within the upper compartment 30 and has two diametrically opposed bleed holes 36 near its bottom end within the bottom compartment 31.

A further short open-ended vertical tube 38 extends coaxially through the top wall and provides communication between the upper compartment 30 above the open top end of the tube 35 and external atmosphere. The top end of the tube 29 is covered with an apertured cap 39 above the top wall 33 and the tube 38 is passed through this.

In use, with the sleeve 21 attached to a motor vehicle exhaust pipe, exhaust gases enter the Pitot tube 22 along the centre line of the pipe 20 via the calibrated jet 24 in advance of the tube 29. The gases enter the upper compartment 30 via the hole 28 and the tube 35 and then leave via the tube 38. Some gas is exhausted into the lower compartment 31 via the calibrated bleed holes 36.

With this arrangement a carefully controlled flow of well-mixed gas is maintained through the equipment and, advantageously, the sensors 34 are exposed to such gas at a pressure close to atmospheric with continuous refreshment of the gas without danger of undue infiltration of atmospheric air.

The circuit of Fig. 6 may be used in substitution for the circuits of Figs. 2 to 5. The sensors are connected (at terminals 40) to operational amplifier measuring circuits 41 powered by a constant current generator 42. A precision power supply 43 provides a controlled output (at 44) for powering the heaters of the sensors. The circuits 41 operate strings of 30 triangular LEDs 45 via respective single chip driver ICs (e.g. an AEG/Telefunken U1096B) 46. Of the 30 LEDs, 29 are used for the measuring display and the 30th is used to indicate over range operation.

The circuitry is designed and calibrated to measure 0 to 5% carbon monoxide and 0 to 400 ppm hydrocarbons respectively. The two parallel rows of diodes 45 indicate instantaneous point by point measured values. The system resolution may be better than 0.175% CO and 15 ppm HC; repeatability may be + one diode step at any part of the range.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiments which are described by way of example only.

Thus, for example, with the arrangement of Fig.

1, in order to avoid collection of moisture in the lower tube 4, this may be heated externally, or a transverse (circumferential) peripheral slit may be provided in the pipe 1 to break up lamina flow adjacent the tube inlet 3.

Claims

1. A method of monitoring the content of one or more gaseous substances in a gaseous stream, which method involves the use of an electronic device which is adapted to undergo a change in an electrical property thereof when exposed to at least one said substance and which is disposed within an enclosure having an opening thereto, and wherein said opening is exposed to said gaseous stream so that said enclosure fills with gas therefrom, and the said electrical property of the said device is monitored.

2. A method according to claim 1, wherein said gas flow into said enclosure is restricted.

3. A method according to claim 2, wherein said gas flows through said enclosure and is exhausted therefrom to the atmosphere.

4. A method according to any one of claims 1 to 3, wherein there are two or more sensors in the same said enclosure adapted to be responsive to different said gaseous substances.

5. A method according to any one of claims 1 to 3, wherein there are two or more sensors in different said enclosures adapted to be responsive to different said gaseous substances.

6. A method according to any one of claims 1 to 5, wherein said gaseous stream constitutes the exhaust emission of a road vehicle internal combustion engine.

7. A method according to claim 6, when dependent on claim 4 or 5, wherein the content of carbon monoxide and of hydrocarbons in said exhaust emission are respectively monitored.

8. A method according to claim 6 or 7, wherein the or each said enclosed electronic sensor device is removably applied to the tail end of a vehicle exhaust pipe to receive said exhaust emissions therefrom.

9. Apparatus for use in performing the method of claim 1, comprising an enclosure having an opening thereto for exposure to a gas stream, an electronic device within the enclosure which is adapted to undergo a change in an electrical property thereof when exposed to at least one predetermined gaseous substance in said stream, and electrical monitoring equipment sensitive to changes in the said property of the said device and operable to produce an output in response thereto.

10. Apparatus according to claim 9, wherein said enclosure is defined within a tube, one end of which has said opening thereto, said tube being fixed externally to one side of a pipe with said opening in communication with a hole in said pipe wall whereby gas from said gas stream flowing along said pipe can enter said enclosure through said hole and said opening.

11. Apparatus according to claim 10. wherein said tube is inclined towards the pipe in the intended direction of gas stream flow.

1 2. Apparatus according to claim 11, wherein said tube is sealed at the end thereof opposite said opening.

13. Apparatus according to claim 12, wherein a flow restricting structure is applied to said opening.

14. Apparatus according to claim 13, wherein said flow restricting structure comprises a sintered metal structure.

1 5. Apparatus according to any one of claims 11 to 14, wherein there are two said tubes at different positions on said pipe each containing a respective said sensor.

1 6. Apparatus according to claim 10, wherein a bent conduit is provided to direct part of said gas stream into said enclosure, said conduit having a leading end with a restricted intake jet thereat within the pipe and an opposite outlet end in communication with said enclosure.

1 7. Apparatus according to claim 16, wherein said outlet end is surrounded by a short tube extending into said enclosure, said outlet end having a discharge aperture at the side thereof so that said gas is arranged to flow between said outlet end and said short tube into said enclosure.

18. Apparatus according to claim 16 or 17, wherein said enclosure has an exhaust outlet thereto.

1 9. Apparatus according to claim 1 7 or 18, wherein said outlet end, said short tube and said exhaust outlet are mutually coaxially arranged.

20. Apparatus according to any one of claims 1 6 to 19, wherein said enclosure contains two said sensors respectively for monitoring different substances.

21. Apparatus according to any one of claims 9 to 20, wherein said pipe has a sleeve at one end thereof for detachable connection of the pipe to a vehicle exhaust.

22. Apparatus according to any one of claims 9 to 21, wherein said monitoring equipment is connected to a blank of visual display devices, such as light-emitting diodes, such devices being arranged to be actuated individually in correspondence with said output.

23. Apparatus according to claim 9, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

24. A method according to claim 1, when performed using apparatus of any one of claims 9 to 23.