How does a gas pump shut off automatically when the gas tank is full?
(Lansing State Journal, December 25, 1991 & August 21, 1996)
We went to the experts for an answer to this question. Floyd Conklin of Lansing Ice and Fuel explained to us how fuel oil delivery trucks know when the tank is full; gasoline pumps use a similar mechanism.
When your gas gauge reads empty, your tank is filled with air instead of fuel. As you add fuel, the air must move out of the tank to make way for the gas. Next time you fill your car at the gas station, take a look at the pump nozzle. You will find a hole about a half inch from the end of the nozzle. This hole is connected to a small pipe that travels through the nozzle to the handle.
When you fill your gas tank, some of the displaced air flows through the hole in the nozzle into the pipe. When the tank is full, the hole will be below the gasoline level and no air will pass through the pipe. A mechanical switch inside the nozzle detects when the flow of air stops and shuts off the gas.
You may have noticed that, after the pump shuts off, you can get more gas in your tank by pulling the nozzle out of the tank a bit. This is because you’re moving the position of the hole relative to the gas level and allowing the mechanical switch to reopen.
Conklin also explained how the replacement of the hole controls how full the tank can get before the pump shuts off.
For example, fuel oil tanks typically hold about 275 gallons of fuel. The filling mechanism indicates that the oil flow should be shut off when the tank is about 25 gallons short of being full to allow for fuel expansion as the temperature increases.
Auto fuel pumps also must be set so they leave a little extra room.