How The Fuel Pump At The Garage Knows When Your Tank Is Full

How the fuel pump at the garage knows when your tank is full

Ever wondered how the automatic shut-off on a pump nozzle works?

NewsLaura Clements Senior Reporter12:46, 10 Aug 2018Updated 12:49, 10 Aug 2018 ,Should you leave a child in the car while paying for petrol,

Filling our vehicles with fuel is such a regular feature of our lives, you probably don’t give it more than a second thought.

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You drive in, park up, put the nozzle in the car and wait for the pump to click and stop pumping.

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But have you ever wondered how the pump knows when to shut off once your tank is full?

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The automatic shut-off is actually a rather innovative mechanical process. It involves a very interesting physical phenomenon, known as the Venturi effect.

What is the Venturi effect?

Thanks to their ability to flow, fluids can be transported through pipes and channels. In such kind of transport, pressure plays a very important role.

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With the modification of pressure, fluids can not only be moved from place to place, but can even be exploited to do some work for us.

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When a fluid moving through a pipe faces a constriction, or narrowing of the pipe, the velocity of flow increases at the constriction, with a corresponding drop in the static pressure.

This principle is called the Venturi effect, after the Italian physicist Giovanni Venturi who discovered it, way back in 1797.

The increase in the speed of flow at the point of constriction means that there is a momentary low pressure created at this point. The pressure at other regions of the pipe is much higher.

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Due to this pressure difference, fluid rushes to the constriction from other parts of the pipe.

If a pipe containing another fluid is connected to the pipe with the constriction, then the vacum created by the fast-flowing liquid at the constriction will draw out the liquid in the other pipe.

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What's inside the pump nozzle?

Next time you go to a gas pump, take a closer look at the pump nozzle. Upon inspection, you will find a small, half-inch hole at the end of it.

This hole is connected to a small pipe that runs into the handle. At a point near the end of its length, let’s call it Point A, the pipe veers off in a different direction to a diaphragm of air. At Point A, there is a Venturi tube through which petrol or diesel flows.

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When your car's fuel tank is not full, air is drawn from the tiny hole (of the nozzle) to the diaphragm due to suction. This goes on until the level of fuel in your tank reaches the level of the hole (or passes the hole). As soon as the hole dips beneath the level of fuel, it starts to suck it up.

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Since the density of oil is greater than air, the Venturi tube experiences a change in fluid pressure. As a result, the diaphragm collapses slightly and an automatic shut-off of the flow of fuel through point A occurs.

The Venturi effect is something you see almost every day

Gas Stoves

Gas stoves work by using an inspirator, which is a Venturi tube that has a constriction and an inlet for air.

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When the gas supply is turned on, pressurized gas enters the tube and passes through the narrow opening. At this constriction, a vacuum is formed due to an increase in the speed of the gas. To fill this vacuum, air rushes in from the inlet and mixes with the gas which is delivered to the stove.

Spray Nozzles

In perfume bottles, pressing of the nozzle causes compressed air to be ejected out of a narrow opening at a high speed. The perfume chamber is connected to this zone, and the low pressure developed causes the liquid molecules to get mixed with the air and get forced out.

Airplanes

The wings of airplanes are designed in such a way that they increase the speed of wind passing over them, thus creating a low pressure area. The air below the wings rushes upwards to occupy this vacuum, which results in an upwards lift being produced, thus enabling flight.