I have recently been working on a vacuum feed fuel system which belongs to a 1929 Delage. These were the first popular type of fuel pump which enabled the fuel tank to be located at the rear of the vehicle instead of the slightly discomforting position for a large quantity of petrol: above the front occupant’s legs. In Britain one of these is usually called an “Autovac” which was a proprietary name, although they were available from a number of different manufacturers around the world. They all work on much the same principle: The engine starts to draw a vacuum through the ‘induction pipe connection’ which lowers the pressure in the ‘inner chamber’. Because the ‘main petrol supply connection’ is common with the inner chamber fuel is then drawn from the fuel tank and fills the inner chamber. The reduction in pressure in this area further tightens the ‘drop valve’ onto its seat. As the inner chamber fills with fuel, the float rises and closes the valve which connects the inlet manifold to the induction pipe connection thereby sealing off the unit from the engine vacuum. At the same time a valve is opened to vent the top of the unit to the atmosphere and the mass of the fuel causes the drop valve to open and drain into the outer chamber. This causes the float to drop and the whole cycle is repeated until the outer chamber is full to the correct level.
It is impossible, of course to guarantee that the vacuum valve will always make an absolutely perfect seal and were this to be the only valve in the system then any tiny leak would mean that the whole unit filled to its brim and then allowed neat fuel to be drawn into the engine. Ingeniously when the vacuum valve is shut, the air vent valve must always be open so this can not happen because as well as allowing the unit to function, this valve also acts as a safety valve.
The one thing that can cause neat fuel to be drawn into the engine is a float which sinks and leaves the unit permanently drawing fuel from the tank. This is exactly what has happened with the Delage which has been consuming a lot of ethanol – rich petrol during a long spell in France. The float in the Delage vacuum fuel feed unit is quite small and is made of cork which was coated with shellac – a material which is readily destroyed by ethanol. The solution has been to manufacture a new float from some ethanol resistant “Nitrophyl” imported from America which has been sealed with fuel – proof “dope” after machining. The only worry I have had is that the car is still in France so I can’t give it a test run before it drives back home.
So – for anyone who has driven a vintage car with an Autovac and wondered what all that clicking is about, here it is: The inner chamber on a test rig. Suction is applied by a brake pump taken from a diesel van and which has been adapted for drive by electric motor.
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