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Originally Posted by Aurora40 I'm not quite sure I understand how air gets into a gas tank as the amount of gasoline goes down. I'd think the only time it gets in there is when you fill it? As the gas level goes down, fuel will vaporize to fill the void. |
You're kidding, right? Oh...you're an electrical guy, IIRC. No, if you pull the gasoline out you're going to reduce the pressure in the tank to subatmospheric, simply because you are reducing the volume of the liquid in the tank. If you want to change the phase of the fluid as you suggest, you'll need to increase the temperature. Tank systems always have breather vents to equalize the tank pressure to atmospheric (within a certain tolerance) to account for liquid level changes and pressure changes caused by temperature changes. This prevents explosion or implosion of the tank, or overflowing, as well as providing a (relatively) constant suction pressure at the fuel pump. In the automotive application, the vapor recovery cannisters are hooked into this system to absorb the gasoline vapors.
It's the same principle as holding your finger over the end of a straw that's in a glass of liquid and pulling the straw out--as long as you keep your finger over the end of the straw the liquid will stay in there--but as you
vent the tank by taking your finger off the end, the fluid will flow out...the same thing would happen in your gas tank if you didn't breathe air into there to replace the volume of gas being pumped out, the tank pressure would go subatmospheric (pulling a vacuum), the pump would cavitate (possibly damaging the impeller)...the car would stall from being fuel starved.