MASCOT November - December 2003

How Does It Work? No 6 - The Fuel Pump Pt 1

The Tool Chest

In the early days the fuel "pump" was good old gravity, with the tank mounted above the level of the carburettor, usually under the scuttle. In Singers, this design was used on Juniors up to 1931, with the filler neck usually protruding through the top of the scuttle. Safety, together with other considerations, dictated that the fuel should be carried lower down and towards the rear of the vehicle, so a method of pumping the fuel up to the carburettor was required. Four main sources of power are used for this purpose - 1 - Air Pressure (from the exhaust or an air pump); 2 - Vacuum (from the intake manifold); 3 - Mechanical (from the engine itself) and 4 - Electrical (from the battery).

1 - Pressure Feed System. The tank is hermetically sealed when in use and pressure is applied to the surface of the fuel, either by utilising the back-pressure from the exhaust, or by means of a small air pump driven off the engine, so that fuel is forced up to the carburettor. As a rule, an auxiliary hand pump is fitted convenient to the driver to provide the initial "priming" pressure and in case of mechanical failure. When exhaust pressure is used, this is fed to the tank via a spring loaded valve set to lift at 2 - 3 psi. Fuel tanks for pressure systems need to be made much stronger than for gravity or vacuum operated systems, and of course must have an air-tight filler cap.

2 - Vacuum Operated System. The most common of these is the Autovac, see Fig 1.

This is mounted above the level of the carburettor, usually on the engine bulkhead, and uses the depression in the inlet manifold to "suck" or draw up fuel from the main tank into a small capacity auxiliary fuel tank, from which it flows by gravity to the carburettor. The Autovac has two chambers and operates as shown in the drawings at Fig 2. When the inner chamber is empty the float falls, opening the valve A to the vacuum pipe and closing the vent B to atmosphere, The suction holds the dump valve E closed and fuel is drawn into the chamber from the tank through pipe C. As the float rises, the suction valve is closed and the vent opened, allowing the dump valve to open, thereby transferring the fuel into the outer chamber. As the fuel level in the inner chamber falls the cycle is repeated. Fuel from the outer chamber flows to the carburettor through pipe D by gravity.

3 - Mechanical Pump. The most common of these is the SU Pump - see Fig 3. This is driven by the camshaft which operates a flexible diaphragm via a lever which pulls it down against a spring, drawing fuel into the pump chamber through a non-return (Inlet) valve. As the cam rotates, the diaphragm is pushed upwards by the spring, forcing fuel through another non-return (Outlet) valve and up to the carburettor. When the carburettor is full the spring maintains a constant pressure of fuel, pumping only what is required to meet the engine's need. Fuel entering the pump passes upwards through a strainer and any dirt collects in the sediment chamber below. When cleaning the strainer it is important to ensure that the cork seal in the cover, and the fibre washer under the retaining screw, are in good condition, as any air leakage on the suction side of the pump will reduce pump efficiency and may prevent it from pumping at all. Most early SU pumps were fitted with a hand priming lever, which saves a lot of cranking the engine if the tank has run dry or if the car has been standing for more than a few days.

In Part 2 we will have a look at how electrically powered pumps work.