How do gasoline pumps work

This shuts down the flow of air to the nozzle, causing it to automatically shut off as a diaphragm closes and stops the flow of fuel. A born-and-raised Jersey girl, Caitlin Moran has somehow found herself settled in Edinburgh, Scotland. See more articles by Caitlin. Learn how a gas pump nozzle actually works.

The nozzle also contains a very small tube that begins just above the open end of the spout. That tube runs back up into the fuel pump handle and as gas flows through the nozzle, the vacuum pressure created by the venturi causes air to be sucked up through the tube. As long as the tank is not completely full, air flows unimpeded up into the handle, and nothing much happens except that fuel continues to pour into your car.

As your gas tank fills, the fuel level rises until it covers the hole at the end of the nozzle. Suddenly, the flow of air stops and the vacuum pressure begins to build — think of what happens when you suck on a straw without putting it in water; if you place your finger over the open end, the flow of air stops and the pressure inside decreases until the straw collapses.

In the nozzle handle, the vacuum pressure builds until it forces a small diaphragm inside the handle to move. That movement triggers a lever that pops the handle trigger, shutting off the flow of gasoline. Pretty clever, eh? No light-emitting diodes. According to Wikipedia, the Venturi Effect is the reduction in fluid pressure that results when a fluid flows through a constricted section or choke of a pipe. Near the tip of the nozzle is a small tube that leads back to an air diaphragm in the handle of the pump.

When you first start pumping gas, the diaphragm is all puffed up and inflated, and air is flowing through the small tube. Once the tip of the nozzle gets submerged in gas as the tank is filling up , gas starts getting sucked up into that little tube. When the little tube gets full of gas which, remember, is more dense than air , there is a pressure change in the pipe. Suddenly, in an attempt to even out the pressure, the air from the diaphragm gets sucked down and out of the pipe. Once the diaphragm decreases in size, it triggers the automatic shutoff within the nozzle.

An easy at home way to see the Venturi Effect is with a drinking straw. Think of what happens when you suck on a straw that's not in a liquid. The air flows through just fine - this is what the little tube inside the nozzle looks like when you first start pumping gas. Now cover the end of the straw.