Fuel prices--BTU's per gal.

"Morgans" wrote in message
...

"Jerry Wass" wrote

Hate to disagree. but there are more BYU's in a GALLON of Avgas than in
a
GALLON of Mogas. Why?? Avgas weighs more per Gallon...

Most fuel type hydrocarbons have nearly the same BTU's per Lb.

I may have to back up off of that statement, and I'll tell you my
(possible)
faulty reasoning.

Generally, the more refined, higher octane fuel will have less BTU
content.

I "forgot" to take into account that the primary way that 100LL gets its
high octane rating is by the use of lead. That throws things off a little
bit.
--
Jim in NC


Very true, although I don't know enough chemistry to understand just why the
fuels with the higher octane number are generally less dense. Fortunately,
the lead is probably a small fraction of one percent of the fuel by weight.
However, in the case of Mogas, the addition of unknown quantities of so
called oxigenates, such as ethanol, the energy content per unit is reduced
by some amount.

When measuring fuel by volume (usually gallons or litres) the ethanol is
said to contain approximately 60% of the energy of gasolene--that is simply
the relationship which I have heard/read consistently repeated. Thus, Mogas
that contains 10% ethanol should contain about 96% as much energy as "real"
gasolene.

When measuring by weight, I suspect that the result is slightly worse. My
recollection from physics class is that an oxigen atom is about the same
size as a carbon atom and slightly heavier; so that the ethanol should be
slightly heavier by volume. However, the oxigen atom contributes nothing to
energy content--since ample oxigen is readily available from the air.
Therefore, with the caveat that my knowledge of molecular structure and
density is not strong; the effect of ethanol on energy content by weight is
probably worse than by volume. As a guess, Mogas that is 10% ethanol by
volume probably contains about 95% as much energy, per pound or per
kilogram, compared to "real" gasolene--despite being more hygroscopic and
(so they say) also slightly more expensive to produce.

Peter