How does the petrol pump know when my tank is full?

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There appears to be some kind of sensor on the pump, but I still can’t understand how it can tell when the tank is full to turn the pump off

In: Engineering

It’s not the exact same , but think how your toilet knows when to stop filling up the tank. Usually done with a Floating ball in a valve , when it reaches the top from floating up it will then cover the orface/valve.

There is a second smaller sensing tube going down along the main fuel filler. This have some small airflow through it from a pump in the handle which again is driven by the fuel flow. When the fuel comes up to this sensing tube it slightly changes the backpressure which will then trigger a mechanical device in the handle that shuts off the fuel flow.

There’s a small inner tube in the nozzle, visible for example in this stock photo: https://media.istockphoto.com/photos/driver-holding-gas-pump-in-hand-front-view-picture-id177130231.

This small inner tube goes up into the handle where it’s connected back to the main petrol flow tube with a small hole. Thanks to something called the Venturi Effect, the fast flow of petrol past this little hole creates a suction force that pulls out air from the little tube to move along with the petrol in the large tube. Think of this as sucking air in through a straw.

If the level of petrol in your tank is high enough, the entrance to the small inner tube will be submerged, making it harder to suck up more air. Imagine sucking air through a straw and slowly lowering it into a glass of water (or just blocking it with a finger) – once you reach the water it becomes much harder to suck it up the straw.

Now this inner tube is also connected to a membrane in the handle that reacts to changes in pressure by bending. Once the petrol hits the tube, the membrane is sucked in and bends (as it is easier to move than sucking up the petrol through the tube), and attached to that is the hook that releases the handle.

There’s no sensors, it’s all mechanical with some clever use of fluid dynamics.