Fuel prices--BTU's per gal.

I think it was 1995, at Oshkosh, I attended a forum on the differences
between auto fuel and avgas. It was presented by a fellow who owned
and operated one of the few refinerys which produce avgas.

IIRC, a refinery turns crude oil into different hydro-carbons. They
are all solvents, and all support combustion. They are ranked from
light to heavy, based on how many hydrogen atoms there are per each
carbon atom. Butane has two hydrogen, and octane has eight. The
heaviest is asphalt, which is the goo left after all lighter fuels are
driven off, has the most energy per lb, but is the most difficult to
ignite.

Auto fuel is a blend which includes the asphalt, as well as butane in
the winter. Avgas has only three components, which do not include
those at either end of the spectrum (ashalt and butane). So avgas
does not leave the residue that old auto fuel will, nor does the
engine start as easily, nor does it produce as much power.
Because the avgas does not have the lighter components, it does not
vapor lock as readily either.

The octane rating compares the tendency of an engine to knock, based
on the speed of the flamespread. Higher octane ratings mean only that
the fuel burns more slowly, as if lead were added. It does not mean
that the fuel has more power, only that engines can have a higher
compression ratio without knock, so the engine can utilize more of the
energy in the fuel. The lowest octane rating, that will not knock,
makes the most power.

If that fellow gives his talk again, you will certainly enjoy the
hour.