If you’ve ever paid attention to the pump while filling up, you may have noticed that it has a small hole located near the tip of the metal nozzle. That little hole is, possibly, the most ingenious piece of the entire set. And it is responsible for the hose “knowing” when to stop adding fuel and stopping on its own with that characteristic click.
What exactly is it. This small hole is located at the end of the pipe (the part that you insert into the tank) and is connected to a thin, secondary tube that runs inside the nozzle parallel to the main fuel line. The nozzle uses the fuel itself that is being pumped to create the effect that activates the automatic cut. So to speak, the little hole does not pour gasoline, but rather breathes air.
How it works. The key is in a physical principle called the Venturi effect. While the fuel flows at high speed through a narrowing of the duct, a low pressure zone is generated that sucks air through that small hole in the tip. The Venturi effect occurs because The density of gasoline is greater than that of airand it is precisely this phenomenon that causes the dispenser to turn off automatically when the tank is full.
The moment of cutting. When the gasoline level inside the tank rises to cover that hole, the tube stops being able to suck in air. When the airflow is cut off, the suction is triggered and creates a vacuum that pulls on a flexible membrane (a diaphragm) housed in the handle of the nozzle. That movement releases a lever mechanism that slams the main valve shut, stopping fuel instantly.
The pressure change causes the diaphragm to “jump”releasing the mechanical lever that closes the valve and ending with a click. And as you may have already noticed, the cut occurs even if you continue to pull the trigger.
100% mechanical. This entire system is purely mechanical. There are no electronic sensors, no chips, no batteries. The handle simply generates a slight vacuum at the tip of the pipe, and if that point becomes clogged, a mechanism closes the valve. It is basic physics applied to this little invention that we use in our routine, and that is capable of detecting even a small amount of fuel, blocking the hole to prevent it from overflowing.
Security and cuts. This system prevents gasoline from overflowing from the tank, something that would be dangerous (risk of fire) and polluting. But its usefulness goes beyond safe filling.
This extraordinary sensitivity is also the cause of those premature and repeated cuts when the jet turns off even though the tank is not full. The most common cause of these annoying cuts is simply a little gasoline splashing back and covers the hole momentarily, activating the mechanism ahead of time. In cars with short filler tubes, a rapid flow can easily flood that column, so the first recommended remedy is usually to reduce the filling rate. The position of the nozzle and the temperature of the fuel also play a role.
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