| Re: [GMCnet] Boiling Point of Gasoline / Alcohol mixtures - WAS: Gas Tank Heat Shields [message #89935] |
Fri, 25 June 2010 21:28  |
Gary Casey
Messages: 448
Registered: September 2009
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Senior Member |
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I'll give it a shot. It truly is a complex question - essentially unanswerable. That's because gasoline has no boiling "point." It consists of a solution of dozens, or maybe hundreds of compounds, each with their own boiling points. Some compounds(known as the "light ends") in the mix boil by themselves at temperatures lower than -20F and some (of course, the "heavy ends") at several hundred degrees. The gas people have come up with at least 2 (probably more) ways to measure the "boiling" characteristic. One measure is the "Reid(sp?) Vapor Pressure (RVP). To measure that, a sample of fuel is put into a closed container and heated to a given temperature. The pressure is measured and that is the RVP. Winter fuel, for example, is blended with a higher percentage light ends and has a higher RVP than summer fuel in order to aid in cold starting. To reduce evaporative emissions and hot fuel handling (read "vapor lock") summer fuel is blended so it
has a lower RVP. The second common measure that I know of is the fractional distillation curve. I'm not sure if it has a formal name. In that method a sample of fuel is heated and the temperature at which it loses 10% of its weight is called the 10% point (original, these chemists). The common fractions are the 10%, 50% and 90% points. One should not confuse fuel behavior with "boiling." Essentially what happens is that the lighter ends come out of solution and form vapor. This takes time - unlike boiling. And once it comes out of solution it will take time to re-dissolve. Example: Heat a pan of water over a stove. At some temperature (measure it) around 150F you will start to see small bubbles form on the surface of the pan. These are air bubbles formed because water can't hold air in solution at high temperature. How to prove it? Shut the heat off and let it cool - the bubbles won't disappear even though the temperature drops, proving
that they are not water vapor. Same thing with fuel.
Hope that helped.
Gary Casey
I am guessing that someone knows the answer to my simple question and that I
don't need to start a fire or worse. What have I done to deserve the brunt
of your attitude?
If we must have it by brand, that would be fine. If it needs to be finer
than that, we need to know. I am beginning to think this is either a trade
secret or classified information, in which case I am really worried.
People are spending a lot of time and resources to solve a problem that we
don't really understand if the boiling point of the fuel at the pump really
can vary between 100 and 400?F.
Regards,
Ken
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