Does gasoline expand?

[Vendicare]

My husband and I were talking today, and I was going to bring up something I had been taught in Junior High, but I wanted a cite first, before making a fool of myself!

My science teacher told us that a tank of gasoline is never sold full, because when you take the first gallon out, the gas expands to fill it up again. Is there any way this could be true? I really can't think so, or a car would never go empty, but stranger things are true, so I thought I would ask!

http://www.ac.wwu.edu/~vawter/Physic...hermExpan.html

[Silas Sparkhammer]

Here is my guess: a tank of gasoline should never be sold completely full, because it might be moved from a cooler place to a warmer place -- for instance, the car might be parked in direct sunlight -- so that the gas would expand, thus putting the tank under internal pressure, which probably isn't very good for it.

(But...does that make any sense? I would think that liquid gasoline would be less subject to expansion than the gasoline fumes in, say, a half-empty tank.)

(Do gas caps have pressure-relief valves built in? I know that when I've driven a long way and my tank is near empty, when I pop the cap there is a "hiss" as the partial vacuum inside is relieved.)

Silas (long way of saying, "I dunno.")

[jimmy101_again]

Originally Posted by Silas Sparkhammer

(Do gas caps have pressure-relief valves built in? I know that when I've driven a long way and my tank is near empty, when I pop the cap there is a "hiss" as the partial vacuum inside is relieved.)

Automotive gas caps used to have a small hole in them to vent excess pressure caused by rising temperature. Modern cars (at least in the US) vent the gas tank into a carbon filled filter. The filter is supposed to absorb the gasoline and keep it from being released to the atmosphere. When the engine is run, some of the air intake to the engine is drawn through the filter canister and the absorbed gasoline is re-vaporized and burned by the engine.

Small gasoline powered motors (like lawnmowers) still use unvented caps. In a lawnmower, the gas tank is usually very close to the engine. When the engine is running it can cause a significant amount of gasoline to evaporate from the fuel tank. Even after a hot motor is turned off, the gas in the tank continues to evaporate at a significant rate until the engine has cooled. If you were to fully fill the gas tank of a hot lawnmower the gas would start to dribble out of the tank a few minutes after the tank was filled and the cap was installed.

Back to the OP. Like Silas said, you should not completely fill a gas tank. The gas will warm up a bit in the tank and expand. That gasoline has to go somewhere and if there is no air pocket in the tank the liquid gas is forced out either the filling cap or into the vapor recovery system.

[Silas Sparkhammer]

Originally Posted by jimmy101_again

. . . you should not completely fill a gas tank. The gas will warm up a bit in the tank and expand. That gasoline has to go somewhere and if there is no air pocket in the tank the liquid gas is forced out either the filling cap or into the vapor recovery system.

Do modern gas pumps fill the tank completely? Or does the shut-off feature on the nozzle prevent the gasoline from rising up into the fill-pipe in one's car?

I always fill until the automatic shut-off stops further fueling.

(When younger, I used to continue to shove a few more fluid ounces into the fill-pipe, but I was naive and silly.)

Silas (still silly)

Most modern gas tanks in cars have an air space at the top that will keep things from getting too much out of hand. I do fill my tank to the top (and top it off), but that is because I am always on my way to work or home, so I will drain at least a gallon (or two if it is my truck) before I get to home/work.

[Alchemy]

Originally Posted by Silas Sparkhammer

Do modern gas pumps fill the tank completely? Or does the shut-off feature on the nozzle prevent the gasoline from rising up into the fill-pipe in one's car?

Modern gas tanks are shaped so that when fuel starts to back up in the fill tube (activating the shut-off feature) there is a void volume of a few gallons at the top of the tank. This allows for evaporation and liquid expansion without building up so much pressure that the fuel tank is in danger of rupturing. A pressure relief valve opens in the unusual case where the pressure in the tank becomes too high. A flap valve allows air to enter the tank to replace the fuel being drawn out by the fuel pump. For the most part, the only time gasoline vapor leaves the tank is when you're filling it.