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Alana's Expanding Gas

I enjoy listening to you loveable boneheads, but the rambling answer you gave Alana about the gas spewing from her filler tube was the ultimate in BOGOSITY! Baffles and trapped air . . . BAH!! I strongly suspect that the day was warm. The gas she just pumped was cool due to underground storage. As Alana left the tank uncapped while she paid her bill, the gasoline simply expanded due to the fact that it was warming up. I’ve experienced the phenomenon several times when filling gas cans for my lawnmower. There’s a reason the “fill line” is well below the top of the can.

the gasoline simply expanded due to the fact that it was warming up

I seriously doubt that the gas would have warmed enough in the few minutes she was paying for her purchase to expand measurably . Eventually it might have, but it would not happen that quickly.

I’ve also seen gas expand in cans as you describe, but only if I’ve accidentally left the can in the sun where it warms quickly. Even then, the liquid gasoline doesn’t change volume enough to erupt out of the can when I remove the cap. What changes more is the pressure on the vapor inside the can. Vapor pressure is a function of temperature, the vapor pressure of the gasoline vapor increases quickly as it warms.

I agree with WesternRoadtripper. It takes a tank of gas more than a couple of minutes to warm up for it to expand in any measurable way.

The volume a liquid like gasoline expands with increasing temperature – well, it isn’t that much. Ever put a cup of cold water and heated it up in a microwave to make some coffee? Doesn’t come out of the microwave hot any more full – at least to the eye – ]=than when you put it in cold, right? That’s why I, like some otheres here, think the OP is probably incorrect.

On the other hand gasses – like air – increase in volume dramatically when heated. Ever put a balloon on a soda bottle and heat the bottle up with your hair dryer? It makes for a good show. It’s much more likely that the air inside the tank heated up slightly – from the exhaust pipe heat maybe, or maybe the newly added gasoline was warmer than the air already in the tank – or like Click and Clack said, the air in the tank was compressed as the gasoline was put in.

In any event, I think it was almost certainly air pressure – not a liquid volume increase – that caused the spill over.

This is what I think happened.

When the tank was full, the tank vent float closed stopping tank venting; the remaining vapors above the fuel began to compress as the incoming fuel climbed up the filler pipe; the pump handle clicked off as it is supposed to do; the fuel in the fill pipe is still moving into the tank so it will further compress the vapors above the fuel; and when the fuel momentum is spent the antifollover back flow valve will close keeping the pressurized fuel from regurgitating up the fill pipe.

The back flow valve probably does not completely seal the fill pipe from back flow. As the car sat with the cap off, the fuel backed up the pipe and overflowed down the side of the car.

Well that is my hypothesis and I am sticking too it.

Bernoulli