How Do Automatic Gas Pumps Know the Tank is Full and Automatically Shut Off?

Answer 1

When the tank gets fuller, there is more resistance at the pump end while pumping. The pump senses the resistance and when it gets to a certain point, the pump shuts off. It is basically a "demand pump".

Answer 2

As the gasoline flows out of the nozzle, the valve mechanism in the handle senses increased resistance (back pressure) the flow. Think of placing your hand over a water faucet how the pressure increases and the water squirts all over. Since the gasoline can only go into the tank or back up the nozzle, the flow back up the nozzle actuates the shutoff valve. I used to have a vehicle that had a sharp bend in the tube from the filler cap down to the tank. Most nozzles were set with low enough pressures that it was difficult for them to work. Basically they sense the fuel pressure against the tube and mistake it for back pressure. The website below is for one of the most common brands of handles and shows a cross section of the nozzle components.

As for the reason handles shut off too soon for our taxi driver, safety is the reason. As with any product, there has to be a built in margin of safety for possible failure modes of the part. What if the weather is hotter or colder and it causes the valve to stick a little more? Also, as I mentioned not all cars fuel filler locations are the same. Sometimes the nozzle is nearly straight down, but in other vehicles it might be closer to horizontal and more prone to overfilling.

Also, as the nozzles gradually wear out, its safer to trip a little sooner. Most of the time we are talking fractions of a gallon. As someone who routinely keeps track of my mileage, I generally don't find my mileage off by more than tenths of a gallon if I don't fuel up exactly the same every time. Sometimes close enough counts.

One final note about safety. If you've ever tried to get gasoline in New Jersey, you'll find there is no self-service gas stations. Apparently, some intellegent consumers decided to catch a few too many gas stations on fire. Even today, I witness people who sit inside their vehicle while fueling, use cells phones and even smoke sometimes. While I don't mind their right to practice natural selection (i.e. survival of the fittest or with the most common sense), I DO mind them doing so when it just might make my life miserable.

But for the record, I think seatbelt laws and helmet laws are unnecessary, although I always wear my seatbelt. I guess the government just feels a need to protect senseless people from themselves.

Answer 3

They don't know. What they sense is liquid splashing against the metal spout of the nozzle, which gives a signal for the pump to shut off. This point in the evolution of gasoline dispensing is what the engineers and powers that be want us to think is a full tank. This is not the case.

If one carefully controls the rate of gas flow by squeezing the handle gently, you will discover that a substantial volume of fuel can be added. In some autos, this can be another gallon or so. In trucks and vans, this added amount can be as much as four or five gallons. It is this deception by means of engineering that causes problems for the consumer.

For example, if you want to calculate a precise figure for fuel milage, one must fill the tank COMPLETELY, so that one can see a static fuel level in the spout. You then drive a certain distance, fill the tank completely again, then do the math. Since different gas pumps dispense gas at different rates, the shut off point differs enough to render any calculations based on the auto shut off useless.

Another problem often occurs in my line of work. I drive a taxicab. When I get the vehicle, it is "full", as gagued by the previous driver. When I turn it in at the end of the shift, I am to turn it in "full". Drivers never take the time or effort to fill the tank to the absolute limit....rather, they fill it until the auto shut off clicks a few times. The problem is that each pump fills at a different rate, and there can be a huge variation in the degree of fullness between one fill up and another. This can go way beyond a splash or two, and it's not uncommon for a taxi that is supposed to be full to be maybe seven eighths full. In this day and age of three dollar a gallon gasoline, this can add up, big time.

Why is this this way? The reason is that the engineers that design both vehicles and gasoline pumps are motivated by arrogance and contempt. They do not believe that the average Joe can responsibly fill his vehicle safely. So, in keeping with the nannyist attitudes that have brought us mandatory seat belt laws, helmet laws, and Q Tips that tell us not to insert them in one's ear (PLEASE!!!!), we have gas pumps and gas tanks that do our thinking for us. And if you are average Joe or Jane Sheep consumer, this insidious bit of "knowing what's best for you" goes unnoticed. If , however, determining exact figures for milage and exact degrees of fullness are part and parcel of your life, you're just out of luck, bubba.

If you're not outraged, you're not paying attention!

Answer 4

you are talking about the gas tank in the car.
The filler tube where we insert the hose nozzle, has a flapper valve. It opens one way down only, and is spring loaded with mechanism that activates the pump off switch,

When tank is full the flapper will try to close, activating the pump off switch.

Answer 5

I'm not sure what the technical term is but there's like a floater switch that is in the nozzle that is triggered when the tank is full. They make the same thing for water hoses for filling up batteries.

Answer 6

because when the gas reaches the tip of the gas pump that tells it that it isn't going to be able to fit any more gas in it.

Answer 7

The end of the nozzle you put into the tank has a sensor on it.

Answer 8

Omg great question I so always wanted to know how that works!

Answer 9

yes

Answer 10

backpressure