FIELD OF THE INVENTION THIS INVENTION relates to a system for controlling fuel supplied through a carburettor to an associated internal combustion engine of a motor vehicle.

BACKGROUND OF THE INVENTION A problem with currently known motor vehicle carburettor fuel supply systems is that when the throttle valve is closed and the vehicle is in motion, unnecessary fuel is burned. This may have the added disadvantage of excessive usage of the brakes especially during rapid deceleration. In Patent Specification SU1364764, there is disclosed a fuel supply system in which fuel is prevented from being supplied to the engine upon closure of the throttle valve. Providing the engine speed is below a threshold level, fuel is restored a short interval after the clutch is activated to disengage the engine. Unfortunately, this system has the problem that the engine may stall if it is running below idle speed and the clutch has not been activated. SUMMARY OF THE INVENTION

It is an object of the invention to overcome or at least alleviate some of the problems of controlling fuel supply to an internal combustion engine via a carburettor. According to one aspect of the invention there is provided a system for controlling fuel supply to an internal combustion engine via a carburettor, the system including:- a solenoid valve associated with a fuel supply for selective control of flow of fuel to said carburettor; a throttle position sensing means adapted to

generate a throttle closed signal indicative of a substantially closed position of a throttle valve of said carburettor; and an engine speed sensing means adapted to generate a first signal indicative of engine speed above a predetermined threshold speed value and a second signal indicative of engine speed below said threshold speed value, wherein the system is characterised in that in use, when both said throttle closed signal and said first signal are generated the solenoid valve is controlled to prevent fuel flow, and whenever said second signal is generated the solenoid valve is controlled to permit fuel to flow to the carburettor.

Preferably, said solenoid valve is an idle solenoid valve associated with said carburettor.

Suitably, the engine speed sensing means includes comparison means for comparing an electrical signal indicative of the engine speed against an electrical signal related to the predetermined threshold speed value.

Preferably, the engine speed sensing means includes adjustment means for adjusting the predetermined threshold speed value.

Preferably, the engine speed sensing means is adapted such that said threshold speed value is between idling speed of said engine and 600 revolutions per minute above idling speed. Suitably, the engine speed sensing means includes a compensation means for increasing the magnitude of the predetermined threshold speed value when a choke means associated with the carburettor is actuate . The throttle position sensing means may be a pressure actuated switch responsive to a change in intake manifold pressure of said engine.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention will be understood and put into practical effect, reference will now be made to the following preferred embodiments in which:-

FIGS. 1a-1e illustrate a first embodiment of the invention;

FIG. 2 illustrates a second embodiment of the invention; FIG. 3 illustrates a third embodiment of the invention;

FIG. 4 illustrates an embodiment of an engine speed sensing means for use in the invention; and FIG. 5 illustrates another embodiment of an engine speed sensing means.

DETAILED DESCRIPTION

Referring to FIG. 1a, there is illustrated a system 10 for controlling fuel supply to an internal combustion engine via a carburettor in which the engine powers a vehicle such as an automobile or motorcycle. System 10 includes an ignition switch 1 in series with two parallel switches 2 and 3. A solenoid valve 4 electrically connected to switches 2 and 3, is controlled such that it is energised whenever one of switches 2 or 3 are closed. When energised, valve 4 allows fuel to flow to the carburettor in which valve 4 is usually the idle solenoid valve associated with the carburettor. When open, switch 2 provides a throttle closed signal to valve 4 which indicates that the throttle valve is in a closed position. Switch 2 may be a pressure activated switch responsive to changes in intake manifold pressure. Switch 3 provides an engine speed threshold signal which, when open, indicates engine speed is above a predetermined threshold speed value TSV and, when closed, indicates

that the engine speed is below threshold speed value TSV.

As illustrated in FIG. 1a, the ignition switch 1 is closed, thereby actuating the ignition circuit of the internal combustion engine. Accordingly, if switch 2 (having normally open contacts) is in an open position, as illustrated, then switch 3 will energise valve 4 only if the engine speed is below threshold speed value TSV. Upon the throttle valve being activated

(opened to increase engine speed) switch 2 closes as illustrated in FIG. 1b. Valve 4 will therefore remain energised whether or not switch 3 is open or closed. Accordingly, fuel will continue to flow to the carburettor when the engine speed is above threshold speed value TSV.

As illustrated in FIG. 1c, when deceleration of the vehicle is required, the throttle valve is closed whereby switch 2 opens. This results in valve 4 being de-energised and therefore no fuel flows into the carburettor. The vehicle then slows down without burning unnecessary fuel. Further, because no fuel is being burnt in the cylinders, an improvement in vehicle deceleration may result therefore reducing demand upon the brakes.

As illustrated in FIG. 1d, whenever the engine speed drops below threshold speed value TSV, switch 3 will close energising valve 4. This therefore allows fuel to flow to the carburettor irrespective of whether or not switch 2 is closed.

When the throttle valve is again activated, as illustrated in FIG. 1e, switch 2 closes. Upon the engine speed exceeding threshold speed value TSV, switch 3 will open and the valve 4 will remain energised due to switch 2 being closed.

Ideally the threshold speed value TSV is selected to be the idle speed of the engine. In

practice, this value may have to be adjusted to be slightly higher than the idle speed and is usually set range between idle speed and 600 revs per minute above idle speed. Referring to FIG. 2, there is illustrated a second system 11 similar to that of FIGS. 1a to 1e. However, switch 5, which is inversely operable to that of switch 3 (i.e. normally open contacts), is electrically connected in parallel with a standard brake light switch 6.

Switches 5 and 6 are electrically connected to brake lights 8, whereby upon deceleration caused by closure of the throttle, switch 5 closes and activates brake lights 8. Similarly, if the brakes are activated, switch 6 closes and activates brake lights 8.

Referring to FIG. 3, there is illustrated a further system 12 in which the normally open contacts of switch 5 are in series with switch 9. Switch 9 performs the same function of switch 2 but it has normally closed contacts. Switches 5 and 9 control energisation of valve 8 which allows fuel to flow when it is de-energised.

In system 12, if either switch 5 or switch 9 are damaged, or there is a break in the electrical connections between the 12v supply and valve 8, the engine will still be supplied by fuel.

In FIG. 4, there is illustrated a preferred embodiment of an engine speed sensing means 13 which control switch 3 and switch 5. Engine speed sensing means 13 has an input terminal 14 which is electrically connected to the points of the ignition circuit for monitoring the frequency of the opening and closing thereof. Resistor R2 is a current li iter and

Capacitor C4 stores pulses which are discharged to the Capacitor C2 during the opening and closing of the

points. Further, Diode D2 supplies Capacitor C2 with negative pulses and Diode D3 provides for Capacitor C2 to discharge through Resistor R3. Therefore, the voltage at the input pin P3 of operational amplifier 0A1 is related to the engine speed. Voltage threshold resistor network VRN comprising R1 , R4, R5, R6, R7, R8, R13, R14, VR1 , VR2 and VR3 provide a comparator threshold voltage, related to threshold speed value TSV, at non-inventing input pin P2 of operatipnal amplifier 0A1. Operational amplifier 0A1 and its associated circuitry VRN and R3 provide a comparison means for comparing the voltage at pin P3 against the threshold voltage at pin P2.

If engine speed sensing means 13 is to monitor a 4 cylinder engine, then switch sw1 is closed. Alternatively, sw2 is closed for 6 cylinders and sw3 is closed for 8 cylinders. Further, a compensation means in the form of a choke activated relay RL2 inserts a resistor R15 in series with voltage threshold resistor network VRN. When activation of a choke associated with the carburettor is detected, relay RL2 is energised and the threshold voltage at pin P2 increases.

Upon the voltage level at input pin P3 reaching a value greater than the voltage at inventing input pin P2, the output voltage of operational amplifier 0A1 will be switched from 0 volts to approximately 10 volts. This voltage is sufficient to provide a current flow into the base of transistor TR2. Accordingly, transistor TR2 will conduct which raise the base voltage of transistor TR1 , therefore transistor TR1 conducts which energises relay RL1.

Relay RL1 controls switches 3 and 5 which are used to energise and de-energise valve 4 as described above. Light emitting Diode LP1 is used to indicate whether or not RL1 is energised and Diode D5 reduces the effect of back EMF across the coil of RL1.

An alternative engine speed sensing means 15 is illustrated in FIG. 5. This uses a standard integrated circuit IC1 LM2907N-8 which energises relay RL1 when the speed of the making and breaking of the points is above threshold speed value TSV determined by a voltage at input pin P7 of circuit IC1. This voltage is determined by resistor network R15, R16 and VR4. • Further, choke activated relay RL2 is included to increase the voltage at input 7 whilst the choke is actuated.

Relay RL2 allows threshold speed value TSV to be increased to compensate for increased engine speed when the carburettor choke is activated thereby preventing stalling of the engine. Relay RL2 can be controlled by an engine temperature sensor or by any other choke monitoring device.

Although the invention has been described with reference to preferred embodiments, it is to be understood that the invention is not limited to the specific embodiments described herein.