How does a gas pump know when my tank is full?

­Your car can sense just how much fuel it needs.
­Your car can sense just how much fuel it needs.

­This mechanism has been around for a long time, so it is safe to say there is not a miniature camera inside the nozzle hooked to a microprocessor. It's purely mechanical -- and ingenious.

Near the tip of the nozzle is a small hole, and a small pipe leads back from the hole into the handle. Suction is applied to this pipe using a Venturi. When the tank is not full, air is being drawn through the hole by the vacuum, and the air flows easily. When gasoline in the tank rises high enough to block the hole, a mechanical linkage in the handle senses the change in suction and flips the nozzle off.

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­Here's a way to think about it -- you've got a small pipe with suction being applied at one end and air flowing through the pipe easily. If you stick the free end of the pipe in a glass of water, much more suction is needed, so a vacuum develops in the middle of the pipe. That vacuum can be used to flip a lever that cuts off the nozzle.

The next time you fill up your tank, look for this hole either on the inside or the outside of the tip.

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Originally Published: Mar 30, 2009

Gas Pump FAQs

How do gas pumps know when to stop?
As gasoline enters the tank, air begins to exit it. Gas pumps stop when there is no more air flowing through the nozzle and the change in air pressure causes the nozzle valve to shut automatically.
Does a gas pump automatically stop when the tank is full?
Gas pumps are mechanically designed to automatically stop pumping gas as soon as the tank is full. The nozzle valve shuts automatically once the gasoline blocks the air in the Venturi tube.
How do you know when your gas tank is full?
If you're planning to fill your fuel tank all the way, you will know the tank is full when you hear a sound indicating that the valve shut off. At this point, you will no longer be able to fill your tank any further.
How does a gas pump nozzle work?
The fuel passes through a Venturi tube in the nozzle, which changes the speed and pressure of the fluid passing through. This creates a vacuum that pulls on the valve and shuts it off when there's no more air passing through.

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