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way, the mixture explodes, rather
than expands smoothly – in other
words, detonation is caused. It's
otherwise known as pinking, because
it sounds like that. You will hear it in
your car if you make it work too
hard (try going uphill in high gear).
Detonation can cause the
temperatures inside the cylinder head
to rise to the melting point of the
components inside it, with the piston
usually going first. The hot gases will
leak past the piston rings, pressurise
the crankcase and blow the oil out.
Net result: seized engine and holes in
the pistons. Time to open the wallet.
Because of these problems, engines
use fuel with an anti-knock additive,
which used to be lead, to ensure fuel
ignites smoothly, and doesn't
explode, and to stop it igniting
before it's meant to (in the days
before carburettors, fuel was much
more volatile, and could be ignited
ten feet away). Lead, of course, is no
longer politically correct so, in cars,
the timing of engines is adjusted to
produce the same effect with
unleaded fuel.
The aviation industry still uses it,
though. The "LL" in 100LL stands
44 Canadian Private Pilot Studies
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.
Anyhow, the octane rating reflects the
ability of fuel to expand evenly.
Aviation fuel is coloured this way:
Colour Fuel
Red 80/87
Blue 100LL
Green 100/130
The smaller the combustion
chamber, the higher the compression
ratio of the engine, which is actually
the difference between the chamber
and the stroke of the piston or, in
other words, the capacity of the
chamber with the piston at each end
of its stroke. Pistons generally have a
concave surface at the top.
Aero engines are usually cooled by
air, using the flow caused by forward
movement, but some have complete
cooling systems if the engine is
packed into a tight space that air
finds it difficult to move through
(the Spitfire, for example, used large
amounts of glycol).
The fins outside a cylinder head are
there to increase the total surface
available to the cooling airstream.
Timing
The cylinder head contains valves
which must be opened and closed at
precise times to allow the fuel/air
mixture in and exhaust gases out (fuel
or inlet valves and exhaust valves):
Valves are hollow, or partly filled
with sodium to encourage heat
transfer.
As it turns, the crankshaft will turn a
smaller version of itself (called a
camshaft, which rotates at half the
speed), linked directly to the valve
rockers at the top of the cylinder with
a long metal rod. The bottom end of
the rod is enclosed in a tappet (to
save wear), and the top end hits the
valve rocker directly, pushing the
valve open. As the engine gets
hotter, these rods expand, so there is
a little clearance to allow for this,
called the valve rocker clearance (valve
rockers are not tappets).
There are reasons why it's a good
idea to open valves at a different
time than top dead centre – one is
that it helps with unleaded fuel, as
Airframes, Engines & Systems 45
mentioned above. Another is that an
engine is complicated, with a lot
happening in a short time, and some
anticipation here and there doesn't
go amiss. Opening early is called
valve lead, and being late is called valve
lag. When open at the same time,
you get valve overlap.
Ignition is automatically advanced as
RPM increases, and retarded when a
light aircraft starts up (the spark is
intensified as well). This is done with
an impulse starter, which uses a coiled
spring, or a low tension booster and
retard breaker.
The Carburettor
This is a device that mixes fuel and
air in the correct proportions,
　
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