The patent Autovac vacuum petrol feed apparatus has for its object the elevation of fuel from a main tank placed below the level of the carburetter, thereby gaining the advantages of accessibility, increased fuel capacity and correct weight distribution.

The system employs a small auxiliary tank usually mounted on the engine side of the dashboard, with its base above the carburetter float chamber. It is divided into two chambers - the inner or vacuum chamber being connected to the induction pipe and main petrol tank, and the lower or reserve chamber to the carburetter. Communication between the two is via the drop valve at the base of the inner chamber.

The engine suction creates a partial vacuum in the upper chamber, thus closing the drop valve and drawing up petrol from the main tank. As the fuel flows in, the float rises. When it reaches a certain height two valves are operated - one cuts off the suction, the other admits air; this admission of air destroys the vacuum, releases the drop valve and allows the petrol to flow into the outer chamber. As the reserve chamber is always open to the atmosphere through the air vent the fuel flows to the carburetter by gravity.

As the float falls with the outflow of fuel from the inner chamber, the valve mechanism is again actuated and the operation of taking in fuel is repeated.

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