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Re: [T3] Wish me luck!


On 19 Dec 2005 at 23:04, Constantino Tobio wrote:

> Your tank is in the shade so it's generally cooler than ambient
> temperature. It is under greater than atmospheric pressure because the
> fuel is vaporizing, which also raises the boiling point. Some of this
> pressure is being relieved by the ventilation system. This vapor is also
> condensing against the walls of the tank, continuing the cycle. Since
> the tank is relatively well sealed your fuel isn't going to all boil away.

Unless there's some defect in your system, the tank is completely vented and no 
pressure buildup is possible.

The gas can only boil as fast as heat soaks into the tank. As the gas boils, it 
soaks up the heat of vaporization, keeping the tank from getting hotter. I 
don't know what the heat of vaporization is for gasoline, but I'm pretty sure 
that if you put a thermometer in your trunk (in the "shade") you'd find the 
temp pretty hot in there when the car is parked in the sun. OTOH, the bottom of 
the tank, which the fuel is actually in contact with, is probably cooler.

> Because gasoline is a volatile organic compound, the molecules have a
> relatively weak affinity to each other- unlike water, which have
> positively charged and negatively charged ends that attract each other
> very well. This is part of the reason why boiling water is much more
> violent affair than boiling gasoline.

To my knowledge, being polar or non-polar does not have anything to do with the 
how violent the boiling might be. That would only have to do with how fast you 
are putting heat into the liquid.

> Ever notice how working with gasoline on a warm day makes for more fumes
> than working on a cold day? I could barely smell the gasoline in the
> fuel tank of my Passat when I changed the fuel pump- it was 40 degrees
> in my garage. When I changed the fuel filter a few weeks earlier, it was
> an unseasonable 65 degree day, and the smell of fuel was very strong.

Absolutely correct. The vapor pressure of any material increases up to the 
point where it boils, which is the temp where its vapor pressure equals 
atmospheric pressure.

> So yeah, fuel might be boiling in your tank on a warm enough day. It's
> just not rolling and bubbling like a teakettle on the stove.

I agree that it's possible. It's certainly possible to have any degree of 
boiling with any liquid. It just depends on the rate of heat flow into the 
liquid.

I'm just skeptical about this 77F figure, especially since none of the typical 
chemicals that I expected to see in gasoline had boiling points anywhere close 
to that low. OTOH, you're the only one who has come up with a reference, and 
that reference looks authoritative.

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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