Aaron Widmar
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The Science Behind Siphoning Gas out of a Car

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siphon gas tank hose science

Siphoning gas isn’t just done by thieves to steal fuel from parked cars. The act of siphoning gas is useful for many legitimate reasons, such as emptying a car’s tank of contaminated fuel or obtaining gas in a power outage. If you’ve never siphoned gas before — or if you have and aren’t sure how it works — there are scientific principles at work you should learn about.

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The basics principles of siphoning a liquid

gas can tank siphon science liquid hose diagramHow are you able to move gasoline from a car’s gas tank to a gas can without mechanical devices? It’s all about gravity, liquid cohesion, and air pressure.

The process involves inserting a hose into the gas tank and placing the other end in a gas can that’s positioned lower than the height of the gas tank. Eventually, the air pressure in the tank will force the standing liquid into the hose and eventually flow through to the other end, creating a cohesive flow of liquid. The gas will continue its flow due to suction and the cohesive nature of liquids. You can speed up the process using a pump to quickly create suction on the exiting end of the hose and get the gas to start flowing sooner.

Newer vehicles feature rollover valves that help relieve tank pressure and prevent back-flow, but they also prevent gas siphoning, so traditional siphoning methods and devices aren’t effective or could even cause damage. To siphon gas from newer cars, you’ll need a modern pump and hose with a very small diameter, but the principle still applies.

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Sources: Wonderopolis, Wired