Fuel Feeds - The Vacuum System

Virtually all new cars now have the fuel fed to the engine by "injection" with or without an engine management system. Catalytic converters in the exhaust system are very susceptible to damage by an engine mis-fire and these can easily come about with a fuel and air mixture supplied to the engine by carburettor, especially when cold.

Over the 100 or so years of motoring a number of differing systems of fuel delivery have been used, the early cars used gravity, the fuel tank being mounted under the drivers seat, or as the with the Model A Ford, the scuttle formed the fuel tank and petrol ran by gravity down to the up-draught carburettor. There was then the positive pump method whereby petrol is drawn from the main supply tank and forced under pressure to the float chamber of the carburettor by means of a reciprocating type of pump. The advantages of this system is that there is a positive supply under all conditions of running, the amount increases with the speed of the engine, no shortage on climbing long hills and it is completely independent of throttle position or vacuum in the inlet pipe.

Petrol pumps used on the more modern motor vehicle engines belong either to the mechanical or electric types. In the case of the former these are driven off the engine cam shaft or timing gear, the pump plunger operating the diaphragm, on a diesel engine the diaphragm type feed pump is built as part of the fuel injection pump. Of the electric type, one of the most common is the S.U. (Skinners Union), this was fitted as standard by Morris, Wolseley, M.G. Riley and a number of other British makes. Basically the diaphragm is attached to the armature and when the magnet is energised it attracts the magnet which pulls the diaphragm with it, thus sucking the petrol through the suction valve, this constant movement is brought about by a set of points working in conjunction with a rocker and throw-over mechanism.

The purpose of this article is however the "Vacuum System", the two main names the Vintage Motorist will come across are "Autovac" which will be found on most British manufactured vehicles and "Stuart" on those made in the United States. The operation principals are however the same. The tanks vary in size depending on their proposed use, the small car probably has an outer tank which holds around one to two pints of fuel whereas that used on the Leyland "Tiger" holds two gallons. When an engine is running light, such as travelling down hill or on the flat considerable vacuum is created, however when climbing up a long hill there is very little vacuum and the engine relies on its supply of fuel from that previously fed to the outer tank. Many older drivers of cars produced by Ford (consuls, zephyrs, etc) will well remember the vacuum windscreen wipers, they rushed across the screen when travelling down hill and almost stopped when going up.

As can be seen from the illustration there are two tanks, an inner or vacuum chamber and the outer or reserve chamber which supplies the fuel by gravity to the carburettor. The inner tank is connected to the vehicles main fuel tank and also the induction manifold of the engine. Communication between the two tanks is by a drop valve. Suction from the engine causes a vacuum in the inner chamber thus closing the drop valve and drawing fuel from the main tank. As the fuel flows in the float rises and when it reaches a predetermined level two valves are operated, one shuts off the suction and the other admits air, the admission of air destroys the vacuum and releases the drop valve which in turn allows the fuel to enter the outer tank. When the float in the inner chamber drops with the emptying of the inner chamber the valve mechanism is again operated and the whole process is repeated. The outer chamber is vented to the atmosphere so as the engine demands fuel it can flow by gravity to the engine.

These vacuum pumps are very reliable and need very little attention, the filter needs to be cleaned occasionally, the sediment chamber draincock opened very more occasionally and a check to see that all joints are airtight. The other great advantage of this pump is that in the unlikely event of failure, the outer tank can be manually filled with anything from one pint to two gallons (depending on autovac type) and the vehicle driven until this supply is exhausted, the process is then again repeated and it is possible to get the vehicle home. Try doing this with a modern car!

All the over 50`s like myself will well remember the half-cab single and double deck buses, well you may not have known it but that "box" adjacent to the near side side-lamp is an "Autovac". Many of the pre-war busses used petrol engines and later diesel units were used. The Autovac thus supplied the carburettor or injector pump in the case of the latter type of engine.