How can the gas pump stop automatically when it is about to fill up?

If you ever use a hose to fill up, you'll notice that when it's almost full, the nozzle will shut off automatically to prevent the car from overflowing , so you have to wonder how they did it. is not?

Next time you fill up with gas, if you shout "full of my dear" then pay attention to the pump,  at the top of the nozzle there will be a small hole about a few millimeters in diameter . This is a straight pipe to the relay inside the fuel pump. Imagine, when you pump liquid into a container, the solution will push the air contained in the tank out. As well as refueling, gasoline (1) from the nozzle flowing into the fuel tank will push the air contained in the fuel tank out into the environment, this amount of air will run into that 1cm small pipe.

Small hole at the top of the gas pump nozzle

1. 1: Flow of gasoline
2. 2: Body of pump hose
3. 3: Air duct
4. 4: The hole diameter is 1cm
5. 5: Relay
6. 6-7: Could it be the pressure sensor?
7. 8: The gas valve opens and closes
8. 9: Trigger pump

Cross section of a gas pump hose

When the gas is full, the air in the fuel tank is no longer available to run into the small tube (3) and the pressure in the hose changes, and now (5) inside the fuel pump knows that the gas is about to fill up so it will Close the valve (8) to prevent the petrol from flowing out, to avoid overfilling the fuel tank. That is why you will see that people will pull out a little bit of gasoline to continue pumping (the space changes, adds air), but if so, even if you press the trigger (9) forever Still no more gas.

This type of relay is only applicable when pumping gasoline into the tank, but if holding the hose and pumping out, the exhaust will always be disconnected. There is also another way is to pump gasoline at the price, enough money, the machine does not automatically disconnect itself.

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