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found that leaning causes lead
oxybromide deposits to cling to
various parts inside the combustion
chamber, which could become
hotspots and cause detonation (the
lead appears as a result of chemical
changes in avgas as it burns). At
richer settings, the lead either doesn't
form or is swept out of the cylinder
(this may be true for lower
performance engines, too).
Don't forget to enrich the mixtures
before increasing power when at
peak EGT or when increasing to
more than 75% power. Move the
engine controls slowly and smoothly,
particularly with a turbocharged
engine. Harsh movements that (on
older engines) will result in a cough
and splutter and having no power
can be embarrassing.
Although many flight manuals state
that as soon an engine is running
without stuttering it's safe to use it to
its fullest extent, try warming up for
a few minutes before applying any
load, at least until you get a positive
indication on the oil temperature
(and pressure) gauges. This ensures a
film of oil over all parts.
Even better, warm it before you start
it, because the insides contract at
different rates – in really cold
Techie Stuff 263
weather the engine may have the
grip of death on the pistons and
cause some strain when you turn the
starter. Equally important is not
letting an engine idle when it's cold,
as it must be fast enough to create a
splash (about 1,000 RPM is fine).
After flight, many engines have a
rundown period which must be strictly
observed if you want to keep it for
any length of time. As engines get
smaller relative to power output,
they have to work harder. Also, in
turbines, there are no heavy areas to
act as heat sinks, like the fins on a
piston engine, which results in
localised hotspots which may
deform, but are safe if cooled
properly, with the help of circulating
oil inside the engine (75% of the air
taken into a turbine is for cooling
purposes). If you shut down too
quickly, the oil no longer circulates,
which means that it may carbonise
on the still-hot surfaces, and build
up enough to prevent the relevant
parts from turning. This coking up
could sieze the engine within 50
hours or less.
If the starter light remains on after
you release the starter button on a
piston engine, you should shut it
down, as it indicates that the starter
is still engaged with the engine and is
being driven by it.
The "LL" in 100LL stands for low
lead, but there is still about four
times more than is needed. As well
as the lead (in the form of TEL—
Tetra-Ethyl Lead), a scavenging agent
(Ethylene DiBromide, or EDB) is also
added to ensure that the lead is
vapourised as far as possible, ready
to be expelled from the cylinder with
other gases. Unfortunately, this is
not 100% successful, but the results
are best at high temperatures and
worst at low ones - the unwanted
extras result in fouling of spark
plugs, heavy deposits in the
combustion chamber, erosion of
valve seats and stems, sticking valves
and piston rings and general
accumulation of sludge and
restriction of flow through fine oil
passages, so it makes you wonder
which is worse (in fact, petrol is not
the only fuel you can use – Japanese
Zeros used to outfly American
aeroplanes because they used ethyl
alcohol). TEL, by the way, is actually
a liquid gas, which was developed by
a subsidiary company (Ethyl, Inc)
belonging to General Motors and I
G Farben sometime before WWII.
In June, 1940, just before the Battle
of Britain, it could only be obtained
through the Anglo-American Oil
Company, or Esso - when the fuel
was changed from 87 octane to 100,
German pilots got a real surprise,
because the Spit could suddenly
climb a whole lot quicker.
Oil
An engine that is not used enough
develops corrosion very quickly on
the inside, and rust flakes, which are
very abrasive, will circulate when the
engine is started, which is why you
have to change the oil even when
you don’t fly a lot. Another reason is
an increased water content, which
will have an acidic effect once it
mixes with the byproducts of
combustion. The most wear takes
place in the first seconds of a cold
start, after the oil has been allowed
to settle. Priming will wash whatever
is left off the cylinder walls, so don't
do too much, and maintain the
　
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