http://www.saflyermag.comaddition of a small amount of Mogas, around 15%, or by priming the
engine with Mogas or Avgas to get it started. Ethanol containing 15%
hydrocarbon fuel has been marketed with limited success as E85
(85% ethanol), particularly for high-compression sports car engines.
An aviation-spec E85 was briefly marketed in the USA under the
name AGE-85.
Baylor University in the USA also obtained
STCs for several aircraft types to run on
straight ethanol, using a separate priming
tank for starting.
However there is still, after
many years, only one aircraft
that is certified straight from the
factory to run on pure ethanol,
and that is the Brazilian
crop-spraying aircraft, the
Embraer Ipanema. High-
ethanol fuels are widely
available in Brazil, which
has a large ethanol industry
producing it from sugar
cane.
Ethanol compensates
to some degree for its lower
energy content by having a
high evaporation coefficient
which significantly cools the intake
temperatures, resulting in greater
thermodynamic efficiency. Since ethanol
has a very low freezing point and in itself is an
antifreeze, carburettor icing is eliminated because the
water vapour is absorbed by the ethanol.
Ethanol has been blamed for being corrosive and attacking
some plastic and rubbers found in fuel systems. South African
research over a period of several years indicates that this is not the
case. It is not the ethanol that damages these materials, but other
by-products of ethanol distilled from fermented biomass. These
other fermentation by-products are mainly ketones such as acetone
and MEK, which wreak havoc on some materials. South African
ethanol comes mainly from Sasol which uses a completely different
process to produce a very high purity ethanol as a by-product from
its coal-to-fuel process.
Mixing ethanol with hydrocarbon fuels does have a few
downsides of which pilots flying on such fuels must educate
themselves.
The main issue is that water does not
mix with Mogas or Avgas, but is readily
soluble in ethanol. Should an ethanol-
blend fuel be contaminated with
water, the water will be absorbed
by the ethanol – up to a point.
Once that saturation point is
reached, the ethanol-water
mix suddenly separates
from the hydrocarbons in
a phenomenon known as
‘phase separation’.
Unfortunately, the
lower the concentration
of ethanol in a blended
fuel (usually 5-10%), the
earlier it will experience phase
separation. The separated fuel
will cause serious engine surging
as the engine alternately takes in
hydrocarbon fuel and an ethanol-water
mix. Indeed, Baylor University managed to
destroy an IO-540 engine in the test cell while
exploring this phenomenon.
Another thing is that mixing an ethanol-blend fuel with other
fuels can radically and unpredictably change the volatility of the end
product.
In summary, ethanol blended fuels are not the end of the
world for aviation, and are even approved by some aircraft engine
manufacturers. However, care should be taken to avoid water
contamination or mixing with other fuels.j