GB2089030A - Liquid flow meter - Google Patents

Abstract

The invention provides a liquid flow rate meter, particularly for measuring fuel consumption, comprising a horizontal or near horizontal measuring tube 12 of uniform cross-section through which the whole or a known proportion of the liquid flow to be measured is caused to flow; boiler means 10 arranged to create in the liquid at a point upstream of the measuring tube, a train of bubbles 20 in such a manner that the bubbles enter the measuring tube in turn and are carried along it in succession by flow through the tube; and means for measuring the time taken by a bubble to travel along a predetermined length of the measuring tube and for deriving from that measurement a signal representing the instantaneous volumetric flow rate of the liquid. <IMAGE>

Description

SPECIFICATION A liquid flow meter This invention relates to meters for measuring liquid flow, and is particularly although not exclusively applicable to liquid fuel consumption meters for automobiles and other vehicles driven by i.c. engines.

For the workshop or workbench testing of i.c.

engines means are available for measuring fuel consumption with considerable accuracy but it is more difficult to measure accurately the instantaneous fuel consumption rate of a vehicle being driven on the road. A meter having this function for fitting to production road vehicles requires to be reasonably accurate, robust and inexpensive and can be used for improving driving techniques in the interest of fuel economy.

With growing interest being shown in the use of "on-board" microprocessors to optimise the varying control parameters of an automobile engine on the road, to improve fuel consumption and control exhaust gas emissions, there is need for such a meter whose output signal can provide valuable data and can be used as an input to the microprocessor, along with inputs from other sensors, to provide feedback control of the fuel/air mixture strength supplied to the engine.

According to the present invention, a liquid flow meter, for example a liquid fuel consumption rate meter for an automobile driven by an i.c. engine, comprises a length of horizontal or near-horizontal tube of uniform cross-section, referred to as the measuring tube, through which the entire liquid flow to be measured is caused to flow, a boiler means arranged to create and inject into the liquid flow upstream of the measuring tube a train of gas bubbles in such a manner that the bubbles enter the measuring tube in turn and are carried along it in succession by the flow through the tube, and means for measuring the time taken by each bubble to travel along a predetermined length of the measuring tube and for deriving from that measurement a signal representing the instantaneous volumetric flow rate of the liquid flow.

Preferably the gas bubbles are of such size that each will occupy the entire cross-section of the measuring tube.

Preferably the gass bubbles are of substantially uniform size.

The volumetric flow rate signal may be displayed as an analogue meter indication or as a digital display, for example in front of the driving seat of a vehicle as an indication of the instantaneous fuel consumption rate, and with simple addition electronic circuitry an indication of the total fuel used during a journey can also be given. The volumetric flow rate signal can also be used as an input to a microprocessor control unit providing a feedback signal for controlling the running of the engine, e.g. for mixture strength control during running.

The train of gas bubbles is preferably generated by a small electrically-heated "boiler" positioned in the fuel upstream of the entrance to the measuring tube, the bubble size being metered by a "spout" through which the bubbles emerge to float upwardly into the measuring tube.

The invention may be carried into practice in various ways, but one specific embodiment thereof will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a diagram of a fuel flow sensor, Figure 2 is a circuit diagram of a typical electronic circuit for timing the passage of the bubbles in the sensor of Figure 1, Figure 3 is a partial view of a modified version of the embodiment of Figure 1, and Figures 4 and 5 are diagrams showing graphically calibration results of the instrument of Figures 1 and 2 with the measuring tube respectively horizontal and inclined upwardly.

The fuel flow sensor illustrated~in Figure 1 is for use as part of a fuel consumption meter for automobiles for use whilst the vehicle is being driven on the road. It comprises a boiler means in the form of a cylindrical container 10 having an entry tube 11 approximately mid-way in its height, through which the entire flow of fuel being supplied to the engine for consumption enters the container. The fuel flow leaves the container 10 via an exit at the top, and travels along a horizontal measuring tube 12 of uniform crosssection on its way to the engine carburettor or fuel injection pump as the case may be. The container 10 will thus be filled with fuel in use, and in its lower part there is mounted a small cylindrical vessel 1 3 with a part-spherical top 14.There are small holes 15 for the circulation of fuel in the side wall of the vessel 1 3 and a downwardly-directed tubular spout 1 6 projects from the side of the vessel 1 3 below the junction of its cylindrical side wall and its part-spherical top 1 4. Screwed into the base of the inner vessel 13 is a small electrical heater 17, for example a flow plug such as is used for starting indirect injection engines, which turns the vessel 13 into a small boiler.

When the heater 1 7 is energised by electrical current, the fuel in the vessel 1 3 is heated causing the evolution of bubbles from the fuel used by the engine. Initially the evolved vapour air and vapour collects in the hemispherical top 14 to the inner member 1 3, when discrete bubbles 20 issue from the downwards sloping spout 1 6 and float upwardly to the top of the outer enclosure 10 entering the measuring tube 12 in succession.

The "boiler" is not in fact a true boiler which raises the temperature of a fluid to its boiling point when the fluid changes its stage (phase) to become a gas. Hydrocarbon liquids are likely to contain dissolved air whose solubility depends on the type of hydrocarbon and the prevailing ambient temperature. The amount of air dissolved decreases as the fluid temperature increases.

By way of example one source of published data gives for a hydrocarbon fuel having a density of 0.85 g/cm3 at 1 5.60C a solubility of air of 12% v/v at 1 0 C, 10% at 200C, 7.5% at 500Cand 5.5% at 800 C. Thus by heating the fuel the amount of air in solution, assuming it is saturated to start with, decreases and is evolved as bubbles.

In the case of diesel fuels the statement in the last paragraph is correct. For petrols, particularly during winter, the somewhat higher solubility of air (circa 17% at 20 C) the bubbles may be enhanced because a certain amount of butane and perhaps propane may be deliberately dissolved in the petrol to increase voiatility at cold starting conditions. Heating of such a petrol will drive off the low boiling point dissolved component as well as any dissolved air. Because of their low boiling point such normally gaseous dissolved components will not condense nor re-dissolve in the short term.

Initially the evolved bubbles may be quite small and be most numerous on or in close proximity to the central heater plug 1 7. They will then rise to the top of the relatively quiescent fuel contained within the spherical top 14, as shown. Breaking the surface of the liquid fuel the bubbles will collect as volume of gas -- mostly air - until the build up of pressure causes a proportion of gas to be forced out through the exit 1 6 against the pressure of the fuel exterior to the vessel 13.The effect of the fuel's surface tension will cause the escaping air to form into a spherical bubble contained within the fuel, with a diameter approximately equal to the exit spout diameter. Se long as the bore of the horizontal velocity measuring tube 12 is not excessively large the diameter of the exit spout 1 6 should be the same diameter or slightly iarger than that of the measuring tube. For the measurement of flows associated with automotive engines the diameters of the measuring tube and the bubble forming spout are likely to be in the range of 2 mm.

Since the air bubbles are derived from the fuel, arrangements must be made to circulate a suitable quantity of fuel within the "boiler" or else the fuel will become vitiated of air and bubble production will cease. It would be convenient to circulate the full fuel flow through the bubble chamber but this would require too much electrical heating energy. It is therefore proposed to use two rows of one or more suitably sized holes 1 5 one row towards the bottom of the outside of the vessel 1 3 and the other towards the top but below the level of the bubble exit holes.By this means fuel enters the vessel 1 3 cold through the bottom holes becomes heated around the electrical heater unit causing air and vapour bubbles to be displaced from the fuel, which then rise vertically upwards into the top spherical cap 14, but as the fuel density drops as it is heated it rises thereby causing thermal convection currents in the fuel and then exits via the top row of holes 1 5. Thus a supply of fuel containing dissolved air and vapour will be continuously available.

To minimise the amount of heating required the vessel 13 should be fairly thick and made of a low conductivity material. Thus most o the electrical energy supplied will heat'- the fuel within the vessel 13.

The upper row of holes 1 5 are arranged to be below the level of the bubble exit spout 1 6 since the gas pressure within the top of the "boiler" will need to be at a slightly higher pressure than at the bubble exit in order to overcome the surface tension of the fuel as the vapour bubble forms as it exits from the "boiler". Depending on the bubble diameter required and the surface tension of the fuel in use the excess pressure is onty a small number of millimetres fuel head, typically 1 mm.

The uniform bore section of the substantially horizontal measuring tube is related to the fuel flow rate being monitored. The bubble size, determined by the size of the exit hole of the spout 16, is arranged such that each bubble 20 locally fills the horizontal tube forming a short plug separatiny the column of flowing fuel to the engine into a series of short lengths between successive bubbles 20 in the tube.

Two light sources shine through the horizontal transparent wall of the measuring tube 12, or through transparent sections in the tube wall, at a fled distance apart along the length of the tube, and their outputs are separately monitored by two photocells 21,21 diametrally opposite. As each bubble 20 passes by, the light transmission through the measuring tube and fuel changes, leading to a step change in photocell output.

The transit time for each bubble 20, provided it completely fills the cross-sectional area of the measuring tube 12, is a measure of the fuel velocity through the measuring tube. Each bubble acts as a free "piston". Knowing the tube section area and the transit time for each bubble between the known measuring reference marks the tirne for a predetermined volume of fuel to pass is determined.

The interruption of each light beam creates a step change in the photocell detector outputs.

These trigger a latch circuit, such as is well known to electronic engineers, creating a square wave form of variable length with time, dependant on the instantaneous rate of fuel consumption.

By known electronic means the time length of the square wave can be measured and displayed to snow the rate of fuel consumption. By using a suitably controlled counter, the volume of fuel used over longer periods of time can be measured.

This can then be related to distances travelled if required, to give kilometres per litre fuel consumption or any other variants required.

Figure 2 shows the basic electronic circuit required to produce the input pulses for a time controlled counter. Here 31 and 31' are Light Emitting Diodes (LED) supplied at a convenient 5 volts. These produce two light beams which pass through the measurement tube 12 diametrally at points 32 and 32', separated by a known fixed distance. Diametrally on the opposite side of the tube 1 2 to the LED's are fixed two photodiodes 21 and 21'. Suppose the fuel with the flow indicating bubbles is flowing from 32 to 32'. As a bubble 20 passes at 32 through the light beam from LED 31 to the photodiode 2 1 there will be a step change in light transmission through the tube 12 as the leading edge of the bubble passes.This produces a pulse from the output of the photodiode 21 which is passed to one input of a "latch" circuit 34 starting an output pulse 35 on the "latch" output side. After a short time interval the leading edge of the same bubble 20 interrupts the light beam from the second LED 31' falling on photodiode 21' causing an output pulse from the latter which then passes to the second input to the "latch" 34, having the effect of cutting off the pulse 35 initiated by the signal from the photodiode 21.

Thus the passing of the bubble 20 over the precalibrated measuring length in the tube 12 has produced a pulse 35 whose period (time length) is the time taken for the bubble to pass from 21 to 21'. If the pulse 35 is now passed to a suitable commercial counter/timer unit, operating in the timing mode, an output signal is obtained which is proportional to the time for the fuel to pass along the measured distance 2121' in the measurement tube.

By known electronic methods the passage time for each bubble 20 between 21 and 21' can be associated with the known calibrated volume to produce a readout for the rate of fuel usage, e.g.

litres/min, in any convenient units.

Again the association of the time to consume the measured quantity of fuel with a signal proportional to a vehicle's road speed permits the display of the vehicle's fuel consumption over relatively short time intervals in litres/kilometre, miles/gallon, or any other convenient units.

To cope with varying ambient air temperatures, changes in fuel composition with season, or widely varying fuel flows known means may be used to vary the rate of heating for bubble generation in proportion to the rate of fuel flow.

In the prototype meter shown in Figure 1 ,the measuring section 12 was a glass tube of 3.2 mm bore with the two photocell sensors 21 spaced 53 mm apart (measuring volume 0.426 cm3). The outputs from the sensors were connected through logic gates to a commercial counter/timer unit, operating in the timing mode. The meter was installed in a test rig containing petrol, delivered through a pump and relief valve, the true fuel flow being measured by a 50 cm3 burette and stop watch.

Figure 4 shows the results obtained during one calibration test using the prototype meter of Figures 1 and 2. The fuel flows are shown along the X-axis, and the ratios of measured to actual flows along the Y-axis.

Due to its inherent nature the sensor will be sensitive to angular orientation of the calibrated measuring tube 12. Inclining the tube of the prototype meter by 7% gave the results shown in Figure 5. As might be expected, the very low flows are significantly affected by the buoyancy of the bubble, whilst at high flows the effect is reduced.

In many environments this effect can be nullified.

For example in a vehicle, the tube could be arranged to be at right angles to the line of vehicle moticn. Alternatively the measuring tube can be bent through 1 800 to reduce the effect.

As described, this fuel consumption meter is suitable for direct use with engines using carburettors. The vapour bubbles 20 can separate out in the float bowl. Where petrol injection is used, or if applied to diesel engines this device will require the use of a suitable "de-aerator" downstream to eliminate any air or vapour bubbles before the fuel enters the fuel injection pump. Such devices are known.

Figure 3, shows an alternative arrangement of the boiler means in which the vessel 13 is replaced by a separate dome 14' and a sleeve 13' both supported in the chamber 10 to provide annular gaps 1 5' at the bottom of the sleeve 13' and between the sleeve 13' and the dome 14' to serve the functions of the holes 1 5 described above. The outlet or exit spout 16' is arranged to be horizontal in this arrangement.

Claims (12)

1. A liquid flow rate meter comprising a horizontal or near horizontal measuring tube of uniform cross-section through which the whole or a known proportion of the liquid flow to be measured is caused to flow; boiler means arranged to create in the liquid at a point upstream of the measuring tube, a train of bubbles in such a manner that the bubbles enter the measuring tube in turn and are carried along it in succession by the flow through the tube; and means for measuring the time taken by a bubble to travel along a predetermined length of the measuring tube and for deriving from that measurement a signal representing the instantaneous volumetric flow rate of the liquid flow.

2. A flow meter as claimed in Claim 1 in which the bubbles are of such size at formation that they are each at least as large as the cross-section of the measuring tube.

3. A flow meter as claimed in Claim 1 or 2 in which the bubbles are of substantially uniform size.

4. A flow meter as claimed in Claim 1, 2 or 3 in which the boiler means comprises a vessel disposed within the liquid of which the flow rate is to be measured; the vessel having an entrance at a lower part and an exit at an upper part for the flow of liquid through the vessel, heater means effective to heat the liquid in the vessel to drive air and/or vapour out of the liqud, an upper cover arranged to form a chamber to collect such air and/or vapour, and an outlet orifice from such chamber to dispense said bubbles from the chamber into the liquid.

5. A flow meter as claimed in Claim 4, in which the outlet orifice is in the torm ot a spout.

6. A flow meter as claimed in Claim 4 in which the spout slopes downwardly.

7. A flow meter as claimed in Claim 4, 5 or 6 in which the heater means comprises an electric heating element.

8. A flow meter as claimed in any one of Claims 4 to 7, in which said entrance and exit for liquid each comprise a row of holes around the vessel.

9. A flow meter as claimed in any one of Claims 4 to 7, in which said entrance and exit for liquid each comprise an annular gap formed around the vessel.

10. A flow meter as claimed in any one of Claims 4 to 9, in which said vessel is contained within a chamber through which the liquid of which the flow rate is to be measured flows.

11. A flow meter as claimed in any preceding claim in which the means for measuring time of travel of a bubble includes a pair of photo electric detector devices arranged one at each end of said length of the measuring tube to give an electrical output pulse signal on the passage of a bubble and an electrical circuit arranged to measure the time between such pulses.

12. A liquid flow rate meter substantially as herein described with reference to the accompanying drawings.