Autovac: The vacuum trick that replaced the pump

In everyday language, a vacuum is the absence of air, or a household appliance that cleans floors by suction. In early automotive engineering, vacuum meant something more specific and useful: the pressure difference created naturally by an engine's intake manifold.

The autovac was a small cylindrical device mounted on or inside an early automobile's engine bay. It tapped into the engine's intake manifold, where the downward stroke of the pistons created negative pressure. This vacuum was routed through a copper line into the autovac chamber, where it lifted gasoline from the main fuel tank below. The fuel rose into a small auxiliary or holding tank inside the autovac itself. Once the auxiliary tank filled, a float valve shut off the vacuum line. From there, gravity alone fed fuel down to the carburetor through a simple gravity line. No pump moving, no pump wearing out.

This was a genuine engineering solution to a real problem. Mechanical fuel pumps, driven by a rotating cam on the engine, were prone to failure. The diaphragm could rupture; the linkages could wear and stick. An autovac had no moving parts inside the engine. It was passive, relying on the engine's own negative pressure to do the work. Many vehicles of the 1920s and 1930s used them, particularly smaller cars where cost and simplicity mattered.

The autovac became obsolete once electric fuel pumps became cheap and reliable, and as fuel tanks were mounted higher on the frame, making gravity feed less practical. By the 1950s, it had nearly disappeared. Today, no automotive trade relies on it, though the principle of using vacuum to move fluid survives in brake boosters and emission control systems.