108 SA FlyerLast month we explored
the basic characteristics
of fuels for aviation
piston engines: energy
content, volatility and
octane rating. In this
part, we will look a
little deeper into those
basic needs for our
engines and consider
some of the issues that
may affect us in the
future.T
HE main driving force
for changes in aviation
fuels is the environmental
imperative that harmful
compounds, mainly
tetra-ethyl lead (TEL), are
removed from aviation
fuels. We have known that this is coming for
a long time, and the days of leaded Avgas
are now nearly over.
Many aircraft have been able to switch
over to ordinary petrol (which us flyers
call Mogas) with little or no modification
required. But even that switch comes with
concerns. Particularly the issue of ethanol
being blended into our automotive fuels.REVISION
Just to recap, the energy content of our
fuel, measured in joules/litre or joules/kg,
is an important factor in aviation. We want
to carry the least volume and weight of fuel
that will carry us the farthest.
Volatility of fuel must be carefully
balanced so that, on the one hand, it
vaporises well enough to mix well with air
for complete combustion. But on the other
hand, it must not be so volatile that under
higher temperatures and lower ambient
pressures it turns into a gas in the aircraft’s
fuel lines, pumps, carburettors and fuelinjection systems, causing vapour locks.
And finally, that so often misunderstood
thing called octane rating, which basically
means that a fuel has a high enough octane
rating to prevent it from igniting under high
temperature and pressure before we want
it to.GETTING TO GRIPS WITH OCTANE
Although the testing of a fuel’s octane
rating in the laboratory is pretty much
the same for each kind of fuel, there are
different methods and different types of
rating systems. This is where the confusion
arises. Aircraft operators and pilots need
to take great care in understanding this
since, if the octane rating is too low, it
can catastrophically and expensively
disassemble your engine.
Using a high octane fuel in an engine
that does not require such high octane
ratings generally will not hurt it. But the fuel
will certainly cost more and, as a general
rule of thumb, higher octane fuels contain
slightly less energy than lower octane fuels,
so fuel consumption will be marginally
higher, plus it will cost more in the first
place.Fuel testing in the laboratory is done
using a special single-cylinder test engine.
The engine is built so that its compression
ratio can be varied. The fuel under test is
run in the engine and the compression ratio
is increased incrementally until the engine
begins to ping or knock from pre-ignition
or detonation – a subject we explored last
month. The compression setting of the test
engine is compared to that of two base-line
fuels that have an ‘octane rating’ of 0 and
100.
It was arbitrarily decided a long time
ago that the baseline ‘zero-octane’ fuel
would be pure heptane and ‘100-octane’
fuel would be pure octane. Or would it?
Unfortunately, although heptane has
the chemical formula of C7H16 (in other
words seven carbon atoms and sixteen
hydrogen atoms joined into a chemical
compound) and octane has the formula of
C8H18, the story does not end there.
The baseline 0-octane fuel is actually
called n-heptane and the 100-octane
reference fuel is an iso-octane known as
2-2-4-trimethylpentane.
So, we will need to dig a bit deeper into
the chemistry of these hydrocarbons.AVIATION FUELS
AOPA BRIEFING - CHRIS MARTINUS
PART TWO
PRESIDENT AIRCRAFT OWNERS AND PILOTS ASSOCIATION – SOUTH AFRICA
& THE FUTURE
Hjelmco Oil in Sweden has been making
unleaded Avgas for many years.