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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
Engines & Systems 45
that it helps with unleaded fuel, as
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,
vapourises it and delivers it to the
cylinders via the inlet manifold (that is,
an inlet that serves manifold, or many,
cylinders). It makes use of the venturi
principle, already discussed in Principles
of Flight, which states that as the
speed of air increases over a
restriction, the corresponding
pressure reduces.
The fuel nozzle, which is connected
directly to the fuel system, is inside
the low pressure area, so the fuel in
the line is sucked out and vapourised
as it is forced to expand (this also
cools the area, so be careful with
carburettor icing, which can form
well in advance of any other type). In
fact, if you could make one small
enough, there's no reason why you
couldn't use an air conditioning unit
to achieve the same effect.
Just before the carburettor ends and
the inlet manifold begins is a butterfly
valve, which is controlled directly by
the throttle. When the throttle is
closed, the butterfly valve is closed,
and vice versa (although better
vapourisation and atomization can
be obtained if fuel is introduced
through holes in the butterfly –
check out the Fish Carburettor, which
was invented to stop fuel wastage
when a car was thrown around).
Even when the butterfly is fully
open, though, there is still resistance
to the flow of fuel from its sideways
presentation. New car engines have
eliminated it altogether by making
the throttle increase the inlet valve
opening time to get the same effect.
　
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