| 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
Karma: 0
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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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| Re: [GMCnet] Boiling Point of Gasoline / Alcohol mixtures - WAS: Gas Tank Heat Shields [message #89972 is a reply to message #89935] |
Sat, 26 June 2010 07:12  |
Ken Coit
Messages: 151
Registered: November 2005
Karma: 0
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Senior Member |
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Thanks Gary. That helps explain my issue at the toll booth in West Virginia
in February. I am certain I bought fuel in even colder country north of
there.
So we have an issue of delivering fuel to the carburetor when there might be
vapors that won't be pumped, and we have an issue of keeping the fuel
already delivered to the float bowl from vaporizing while we pause for a
toll, a light, or whatever. It seems that we might also have a problem
climbing grades while underway.
My coach is still not on the road so I am limited in what I can do
experimentally. However, it sure would be nice if there were a few who were
adventurous enough to add some instrumentation to their rigs in the form of
fuel pressure and temp at the inlet to the filter body or carb if you don't
use one. Knowing the temps back towards the tanks would be useful too, but
this would be a good start to learning whether we are delivering hot fuel or
creating it in the float bowl. A themocouple on the fuel line away from the
intake and exhaust manifolds would be good too.
A meat thermometer with interchangeable thermocouples would be handy and I
will find one. Here's an example:
http://www.thermoworks.com/products/low_cost/oven_temp_timer.html
Thermocouples are available on eBay for $5, including shipping from China
and they deliver fast by air in my case.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=260590374572
I will pull that together and see if it is a reasonable way to get cheap
instrumentation.
--
Ken Coit, ND7N
Raleigh, NC
Parfait Royale
1978 Royale Rear Bath, 403, 3.07
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