How do automatic gas-pump nozzles know when to shut off?

gas tanks, gasoline
Dear Tom and Ray:

How do automatic gas-pump nozzles know when the car's gas tank is almost full, and therefore when to shut off? This question has bothered me since I was 16 and gas was two bits a gallon -- neither of which is true anymore. -- Ross

RAY: Great question, Ross. The nozzle uses a simple mechanism that's been around for decades.

TOM: Basically, there's a little hole near the end of the nozzle. You can look for it the next time you fill up. And attached to that hole is a tube that's connected to the handle.

RAY: When gasoline is flowing freely (i.e., when the tank is not full), the moving liquid creates a vacuum as it pours into the tank, and air gets sucked freely through that tube. But as the tank gets full, the vacuum is reduced.

TOM: And there's a mechanical, vacuum-activated switch in the handle that -- get this -- senses when the vacuum reaches a critical low point and then switches off the gas flow.

RAY: This system is far superior to the previous method used to determine when the tank was full. My brother remembers using that system.

TOM: Yeah, when you felt the gasoline trickle down your pant leg into your shoes, you knew it was just past time to stop squeezing the handle.
Tags (Browse All)
gas tanks, gasoline

You must be logged in to leave a comment. Login / Signup
Support for Car Talk is provided by:

Donate Your Car,
Support Your NPR Station

...and get a tax break!

Get Started

Find a Mechanic

Promo tile

Rocket Fuel