MASCOT January - February 2004

Why How Does It Work?- No 7 - The Fuel Pump Pt 2

from the Tool Chest

In Part I we looked at mechanically powered pumps. Now let's see what we can find about electrically powered ones.

Electric Pumps -1. One of the earliest of these was the Petrolift made by the SU company - See Fig 1. The mechanism consists of a metal tube which contains a hollow iron pump plunger. The plunger has a non-return disc valve at the top and there is another disc valve at the base of the metal tube. The plunger is drawn upwards by a solenoid wound outside the lower part of the metal tube. This action sucks fuel from the tank into a lower chamber. The plunger then drops under gravity and the fuel passes through the plunger and upper disc valve, which prevents it from flowing back. Above the head of the plunger is a light iron sleeve which rests on the plunger and can move up and down with it. The sleeve can also be lifted by a cork float via a piston rod which passes through the float. The rod can move up and down within the float, and has a boss at the top, so that when the float rises it carries the rod and sleeve with it. The purpose of the sleeve is to operate the contact breaker points, which are mounted on a rocking linkage on the outside of the tube.

Opposite the points is a permanent magnet and when the plunger is at the bottom of its stroke the iron sleeve completes the magnetic circuit with the rocking linkage which then pulls the contacts together. This energises the solenoid which raises the plunger, transferring the fuel above it to the float chamber, from where it flows out under gravity to the carburetor, whilst at the same time drawing in more fuel from the tank. As the plunger reaches the top of its stroke, the magnetic circuit is broken and the plunger falls. When the carburetor is full, the float chamber fills and raises the float, which then holds the sleeve in the "up" position, thus keeping the contacts open. As fuel flows into the carburetor, the float and sleeve gradually fall until the magnetic circuit is re-made and the operation of pumping begins again.

Electric Pumps - 2. After the Petrolift came the SU Pressure Pump, see Fig 2, vrhich is widely used on British cars. It works on the diaphragm method similar to the mechanical pump, but is operated by an electromagnet in its main body. An armature is secured to the diaphragm and when power is applied, the magnet pulls the diaphragm in against a spring, creating a partial vacuum in the pump chamber, thus drawing fuel up from the tank past the Inlet Valve through a gauze filter. The Armature Rod passes through the pump body and is screwed into to the contact-breaker points mechanism, which comprises two rocking levers and a moving, or "Throw-Over" Contact. When the armature reaches the end of its suction stroke, the linkage goes over-centre and throws the moving point out of contact. The electromagnet is thus de-energised and the diaphragm returns under the pressure of the spring, forcing fuel past the Delivery Valve to the carburetor. When it reaches the end of its pumping stroke, the over-centre mechanism operates again, this time throwing the moving point over and into contact, re-energising the magnet and re-starting the cycle The armature is held central in the main body by a series of brass rollers which allow freedom of movement in the longitudinal direction. When the carburetor if full, fuel pressure is maintained by the spring, and the pump only operates when the pressure drops as the armature reaches the end of its pumping stroke and the points are thrown back into contact, thereby re-energising the electromagnet and continuing the pumping process.

Normal maintenance consists of removing the filter to clean it and drain any sediment from the pump chamber. Occasionally the points may need cleaning. This is best achieved by inserting a piece of card between the points, then holding the points closed as the card is withdrawn. If it is necessary to replace the diaphragm, it is important to ensure the over-centre mechanism is properly adjusted. This is achieved by screwing in the armature rod until the mechanism just fails to throwover when the armature is pressed home against its spring, then unscrewing it 2/3rds of a turn (4-screw-holes). Before tightening the screws securing the body to the pump chamber, connect the power supply and hold the contact points closed - this will ensure the diaphragm is pulled in and is not under tension as the screws are tightened.