Dec 252011

With the body shell and engine sent away for restoration, attention turned to the refurbishment of the electrical components. The first was the fuel pump which was looking slightly worse for wear and somewhat corroded after 40 plus years.

The SU AUF 300 fuel pump used in the Series 2 E-Types is a fairly simple, displacement type pump, developing up to 2.7psi and a flow rate of 2.4 pints per minute. At its core is a pumping chamber sealed at one end by a flexible diaphragm. Movement of the diaphragm enables the volume of the chamber to be increased and decreased.

SU AUF300 Fuel Pump

When the diaphragm is drawn away from the pumping chamber, the volume of a chamber increases causing a decrease in pressure and fuel is drawn into the chamber. Conversely, when the diaphragm then returns to its original position, the chamber volume is decreased resulting in an increase in pressure, which expels fuel from the chamber.

The pump body actually comprises of three chambers: an inlet chamber, the pumping chamber and the delivery chamber. The inlet and delivery chambers are connected to the pumping chamber by non-return, inlet and outlet valve assemblies. Therefore the passage of fuel is restricted to flowing into the pumping chamber from the inlet chamber only and from the pumping chamber into the deliver chamber. A one-way street!

The inlet and delivery chambers also provide a secondary function of smoothing the flow of fuel.

The AUF300 pump uses a coil housing and sprung armature & spindle assembly to control the movement of the diaphragm. The coil housing is attached to the main pump body and, as the name suggests, contains a wire coil that can be energised when power is supplied.

In an energised state the coil produces a magnetic field which acts on the armature, pulling it and the attached diaphragm toward the coil housing, increasing the pumping chamber volume. Thus petrol is drawn from the fuel tank, via in inlet valve assembly into the pumping chamber.

However when the power is removed, the armature spring pushes the diaphragm back towards the pump body. This forces fuel from the pumping chamber, via the outlet valve assembly, into the delivery chamber and on to the engine bay.

It is worth noting that the pump pressure is dependent on the force applied to return the diaphragm to its original position. Therefore fuel pressure generated by the fuel pump is largely determined by the strength of the diaphragm return spring.

The armature spindle passes through the centre of the coil housing to a contact point assembly. It is the movement of the spindle and the connected contact points that controls the power supply to the coil. When the diaphragm is in its original position, the contact points are closed allowing current to flow to energise the coil. The armature is drawn into the coil housing until the attached spindle pushes the contact point rocker assembly to ‘throw over’, opening the points and removing the electrical current to the coil. As the coil is no longer energized, the magnetic field acting on the armature is lost and the armature spring returns the diaphragm to its original position. The cycle then repeats. The throwing over of the points gives the pump its characteristic ‘tick tick’ sound.

 Posted by at 12:19 am

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