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How Does a Fuel Pump Know When to Stop?

gas pump shutoff

January 24, 2013 - You ever wonder how fuel pumps at the gas station know when your gas tank is full?  Is there some sort of magical leprechaun in your tank that shouts up to a gremlin in the fuel pump, letting him know the tank is all full? Although, mystical automotive creatures would make the mechanics of automobiles more interesting and a blog about auto parts more Tolkien, this is not the case.

The automatic shutoff can be traced all the way back to 1939, so it is not the result of some modern nanotechnology. It is a simple system that relies heavily on vacuum. I am not talking about the vacuum that comes by when you lift up your legs so it can suck up the Cheetos lying beside your feet. The vacuum in a fuel pump nozzle is a Venturi effect. The effect is the reduction in pressure which results when a gas or fluid flows through a pipe, or in this case a nozzle. The next time you are at the pump look at the tip of the nozzle. You will see a hole on the side somewhere. This is where the Venturi port is located. It creates the suction of the air in your gas tank.

When you get to the gas station your tank is empty so there is air in it. When you fill up your tank with gas this air must leave the tank. It goes from your tank and into the gas station system, keeping everything balanced. Once you put enough fuel in your car for the fuel level to reach the Venturi port it can no longer pull in air; it is blocked by fuel. This then creates a pressure imbalance. The imbalance flips a switch that shuts off the pump, and you don’t get gas all over your Nikes!

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