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First of all, the energy content for regular, mid-range or
high octane gas is identical, for the simple reason that
it is all the same fuel. Only change is the additive quality.
High octane fuel uses a more expensive additive to make
it more knock resistant. Should not a better quality additive
be used but rather more of a lower quality additive, then the
energy content of high octane gas can be lower.
If the fuel uses O2 additives the energy content drops which
is the case for high altitude fuels for atmo engines. These
only have 85 octane or so because there is no risk of knocking
as the low ambient air pressure reduces internal pressures
in the cyl at max compression proportionally to the drop
in air pressure. ie. a car that has 11:1 at sea level can
drop to 9:1 or less at higher altitudes.
The 02 additives are being used because as the O2 content
drops many carburated engines would run a mix that is too rich
for the altitude, the O2 additive bringing some missing O2 and
reducing these problems on engines with rudimentary fuel systems.
Technically speaking the O2 additive would not be needed if the
engines could sense air O2 content like the T7 can during combustion.
Because of the fact that the Aero compensates for drops in air
pressure and O2 content through the use of a lot more boost, an
Aero will always run best on high octane fuel when it comes to
maximum acceleration. Under cruise the lower air pressure reduces
drag which then can show up in very good MPG figures. And under cruise
the octane level is not as important as compression is low.
Put quite simply you get 85 octane fuel in high altitude areas
because of all the crap engines that are around. If everyone
had an engine simmilar to the Aero they would sell premium.
In WW2 the Air-Force used GAS which offered
an octane rating of 132 under rich mixture. This gas offered
mainly a better climb performance for the planes as they
could run higher boost at lower altitudes. In fact the
venerable R.R Merlin engines often allowed for OVERBOOST
if this fuel was being used. Under these OVERBOOST conditions
they could provide a lot more HP for a short time at lower altitudes then what
their rated altitude performance was. ( Rated altitude performance
is the altitude at which the engine develops most HP, depending
on the planes intended application, this could vary from low to high
altitudes by using different combinations of superchargers )
I hope you now understand why high octane fuel is interesting for
the AERO, as its Turbo system will adapt boost to altitude.
Regards,
Coolknight
CQFD
, Thu, 15 Feb 2001 13:56:49
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