Here's the nerdy answer to your question:
Gasoline Blending 101.
There is really hardly any propane in gasoline and refiners don't
specifically put propane into gasoline (there is a small amount of propane
in the butane that is blended into gasoline - see next paragraph). Propane
does have a high octane of 104.6, but the RVP is 190 psi (RVP - Reid Vapor
Pressure - is a measure of volatility - a higher number means it vaporizes
more readily). The maximum RVP allowed for gasoline is around 7 to 12
depending on the location and season. For this reason it doesn't take much
propane in gasoline to make the whole blend exceed the RVP limit.
Refiners do blend butane into gasoline ranging from less than 1% in the
summer to 5% in the winter (these are ballpark numbers for California). You
can get more money for butane being sold in gasoline instead of selling it
as butane. The blending strategy is to blend butane into gasoline up to the
gasoline RVP limit. Butane does have a high RVP of 72 for iso-butane and 52
for normal-butane, but that is much lower than propane. The octane is 99.5
for iso-butane and 91.7 for normal-butane so some butane vaporization would
slightly change the octane of the gasoline. However, there is so little
butane in the gasoline in the summer when it's hot out and in the winter
it's colder so vaporization is much less and the butane octane is close
enough to the whole gasoline octane that I think you'd be hard pressed to
measure the difference in octane or notice a difference in engine
performance.
Hope that helps.
Bob
79 Spit
Date: Tue, 5 Oct 1999 15:19:00 -0500
From: rtriplett@bjservices.com
Subject: the vapors
> > You can eliminate the carbon canister, but you will have to run the
> > vent line to somewhere that it won't vent fumes inside the car. And by
> > all means do not plug it up. The car will soon stall when a vacuum
> > develops inside the fuel tank.
Somebody correct me if I'm wrong, but don't refineries nowadays add propane
to
gasoline in order to raise or lower the octane rating of fuel (its cheaper
that
way)? Since propane is normally found as a gas, I just wonder what affect
venting a gas tank to atmosphere might have on its octane rating over time.
Since I normally drive my TR only occasionally, I always get fuel at the
time I
drive it, just enough to cruise around some. This way I keep fresh fuel
when
I'm driving. I wouldn't recommend keeping a low tank for everyone, but I
live
in the desert, so humidity is no problem
|