曝光台 注意防骗
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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
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.
Anyhow, the octane rating reflects the
ability of fuel to expand evenly.
When stored for long periods, the
tendency will be for the octane
rating to decrease slightly if there is a
lot of evaporation (exam question).
Also remember that it isn't the fuel
that burns, but the vapour given off
from it - a lot of fuel is actually
wasted, even in modern engines,
because the fuel droplets going into
the engine are not fully vapourised.
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, but they
can be convex (in cars, anyway) to
artificially reduce the chamber when
the piston stroke isn't long enough,
though it is an aftermarket solution.
Aero engines are usually cooled by
air, using the flow caused by forward
movement, but some have complete
202 Canadian Professional Pilot Studies
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
purely there to increase the total
surface area 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).
Newer engines have an overhead
camshaft, so called because it sits on
top of the engine and opens the
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