GB2043255A - Fuel consumption meter - Google Patents

Abstract

A fuel consumption meter for motor vehicles uses pulse signals from a fuel pump, to indicate rate of fuel consumption and from a speedometer to indicate distance travelled. The speedometer signals 10 are integrated in C1 amplified at 11 and charge C3 via diode pump D1, D2. The fuel pump signals 12, derived across series resistor R10, are used to periodically discharge C3 via switch TR1, TR2 thus producing a sawtooth voltage which is amplified at 13 after passing through low-pass filter R1, C4, R2, C5, R3, C6. The mean value of this voltage represents the ratio between the frequency of input 10 and frequency of input 12 and is indicated on meter 14 which is suitably calibrated and set by VR1, VR2 and VR3. <IMAGE>

Description

SPECIFICATION Fuel consumption meter for use in an automobile This invention relates to a fuel consumption meter for use in an automobile to give at any instant an indication of the distance travelled for each unit of fuel used.

It is useful to have in an automobile a meter giving at any instant a direct indication of fuel usage relative to distance travelled since such a meter enables the automobile to be driven with the economy required, and also enables the engine of the automobile to be tuned to give the balance between economy and performance required.

According to this invention, a fuel consumption meter for use in an automobile to give at any instant an indication of the distance travelled for each unit of fuel used, comprises a first transducer capable of providing a first pulse signal having a repetition frequency proportional to fuel usage; a second transducer capable of providing a second pulse signal having a repetition frequency proportional to distance travelled; processing means to which the first and second pulse signals are supplied, and which in response thereto provides an analogue output signal proportional to the ratio of the pulse repetition frequencies of the first and second pulse signals; and an indicator means to which the output signal is supplied and arranged to indicate directly the distance travelled for each unit of fuel used as represented by said ratio.

The first transducer can be constituted by an electric fuel pump as fitted as standard equipment to same automobiles and as can befitted in place of the otherwise provided mechanical fuel pump in other automobiles.

Such a fuel pump utilizes electric current in short pulses for its operation, and the pulse repetition frequency of such usage is proportional to the rate of flow of fuel through the pump during normal driving conditions.

Otherwise a purpose-built transducer can be used as the first transducer, such transducer being connected into the fuel supply line of the automobile.

The second transducer can be constituted by a suitably modified speedometer of the automobile. Most speedometers utilise a rotating magnet for coupling the speedometer dial to the cable coming from either a road wheel or the gearbox of the automobile, and such a speedometer can be modified by the provision of a fixed coil in the field of influence of the magnet, in which coil a pulse signal having a pulse repetition frequency proportional to the speed of rotation of the magnet, is formed by electromagnetic induction.

Otherwise the second transducer can be constituted by the contact breaker points of the distributor of the automobile although this has the disadvantage that the meter can only be calibrated to give a direct reading when the automobile is in a single predetermined gear unless more than one scale is shown on the meter.

This invention will now be described by way of example with reference to the drawings in which: Figure 1 illustrates the manner of modification of a conventional speedometer to form the second transducer of a meter according to this invention; and Figure 2 is a circuit diagram of a meter according to this invention.

Figure 1 shows the inside of a conventional speedometer in which a cable 1 from a road wheel or the gearbox of an automobile rotates a magnet 2. The magnet is air-coupled to a drag disc 3 to which the indicator needle 4 of the speedometer is connected in order to give an indication of the speed of the automobile.

As shown, the speedometer is modified to form a second transducer for use in a meter according to this invention, by the provision of a small coil 5 on the back-plate 6 of the speedometer in the field of influence of the magnet 2. As the magnet 2 rotates a pulse signal is induced in the coil 5, the repetition frequency of the pulse signal being proportional to the speed of, and thus the distance travelled by, the automobile.

Referring now to Figure 2, the output pulse signal from the speedometer of Figure 1 is fed to an input 10 of a processing means of which Figure 2 is the circuit diagram. This pulse signal is integrated by a capacitor C1 connected across the input 10 and the internal resistance of the coil 5 (Figure 1) and is then fed to a 741 operational amplifier 11. This amplifier 11 is saturated thereby and provides a square wave output signal having a frequency equal to the revolutions per second of the cable 1 of the spedometer (Figure 1). The output signal from the amplifier 11 charges a capacitor C3 through a capacitor C2 and a diode pump formed by diodes D1 and D2.

The processing means has a record input 12 to which the pulse signal from a first transducer, for example an electric fuel pump as previously described, is supplied. This input pulse signal from the first transducer causes transistor TR2 to conduct at the repetition frequency of the input signal, this causing LED1 to flash at this frequency to give a visual indication of the fuel consumption rate as an absolute value not related to distance travelled.

When transistor TR2 conducts, transistorTR1 is rendered conductive and capacitor C3 completely discharges.

Thus, a sawtooth voltage is generated across capacitor C3, the mean value of which is proportional to the ratio of the pulse repetition frequency of the input signal at input 10 from the speedometer second transducer, to the pulse repetition frequency of the input signal at input 12 from the fuel pump first transducer.

The low pass filter constituted by resistors R1 and R4 and capacitors C4 to C6 produces a steady voltage from the sawtooth voltage, which steady voltage is fed to a further 741 operational amplifier 13 which has a variable feed-back resistor VR1 which can be used to control the gain of the amplifier 13 and thus the sensitivity of the meter.

The output of the amplifier 13 is fed by way of a variable resistor VR2 which can also be used to control the sensitivity of the meter, to an indicator means 14 in the form of a suitably calibrated meter by which the required indication of distance travelled for each unit of fuel used, is given.

For initial calibration of the above described meter the number of pulses in the signal from the fuel pump, counted by means of LED1, is determined for a given fuel output from the pump, collected in a measuring container. The fuel throughput per pulse (P) can then be calculated and used for calibration of the meter 14.

The calibration constant (S) for a speedometer is usually printed on the face of the device.

The required sensitivity setting for the meter can then be calculated from the following equation: Reading of meter in = Sensitivity constant (C) x Repetition frequency distance/fuel unit of input signal from (e.g. miles/gallon) modified speedometer Repetition frequency of input signal from fuel pump 1 1 where C = 4542 x S x p (S revs/mile, P miles/pulse) for the circuit shown.

SP A linear scale read from a pointer can be attached to VR1 and can be calibrated in terms of values of the sensitivity constant (C) by the simultaneous injection of suitable test signals of known pulse repetition frequencies into the inputs 10 x 12 and observation of the meter reading. Then by adjustment of VR1 by reference to the scale, the meter can be set for operation to the required sensitivity (C), as determined.

Claims (9)

1. A fuel consumption meter for use in an automobile to give at any instant an indication of the distance travelled for each unit of fuel used, comprising a first transducer capable of providing a first pulse signal having a repetition frequency proportional to fuel usage; a second transducer capable of providing a second pulse signal having a repetition frequency proportional to distance travelled; processing means to which the first and second pulse signals are supplied, and which in response thereto provides an analogue output signal proportional to the ratio of the pulse repetition frequencies of the first and second pulse signals; and an indicator means to which the output signal is supplied and arranged to indicate directly the distance travelled for each unit of fuel used as represented by said ratio.

2. A fuel consumption meter as claimed in Claim 1, wherein the first transducer comprises an electric fuel pump.

3. A fuel consumption meter as claimed in Claim 2, wherein the fuel pump is an impulse type wherein enegising current frequency is proportional to fuel delivery rate.

4. A fuel consumption meter as claimed in any preceding claim, wherein the second transducer comprises a vehicle speedometer suitably modified.

5. Afuel consumption meter as claimed in Claim 4, wherein the second transducer comprises a coil pick-up cooperating with the speedometer drive magnet to produce an output pulse rate proportional to vehicle speed.

6. A fuel consumption meter as claimed in any one of Claims 1 to 2, wherein the second transducer comprises a connection to the vehicle ignition distributor contacts.

7. Afuel consumption meter as claimed in any preceding claim, wherein the second transducer output is integrated and squared to produce a square wave output of repetition rate proportional to input frequency, said output charging a capacitor through a diode pump, the first transducer output being coupled to switch means to discharge the capacitor on occurence of each input pulse whereby a sawtooth voltage of mean value proportional to the ratio between the pulse frequency derived from the second transducer and the pulse frequency derived from the first transducer.

8. Afuel consumption meter as claimed in Claim 7, wherein the voltage across the capacitor is fed to an amplifier through a low pass filter, the amplifier output being connected to indication means.

9. A fuel consumption meter constructed and arranged to function substantially as herein described with reference to the accompanying drawings.