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an entirely wrong time and strain the
crankshaft. Instead, the cables are
arranged out of order, on a fourcylinder
engine as 1342 or 1243 (you
will often see these numbers
moulded into the cylinder head as a
reminder, in case you were
wondering what they were for).
The switches in the cockpit ground
magnetos to Earth through the
primary circuit, because they cannot
be switched off (they work as long as
they spin), so they must always be treated
as live. Ground connections can fail.
The 4-Stroke (Otto) Cycle
Now we are acquainted with them,
here, in excruciating detail, is a
description of the movements of all
the parts in an engine.
It all starts with the piston at top
dead centre, ready to move down
and suck in a fuel/air mixture from
the carburettor, through the fuel
valve, which has just opened:
On top of the suction created by the
piston's downward movement,
atmospheric pressure helps to force
the fuel and air in (when it is less, the
supercharger or turbocharger helps).
The valve closes as the piston
reaches bottom dead centre, so the
chamber is filled.
With both valves closed, the piston
starts moving up again, compressing
and heating the mix, as well as
increasing the density, which helps the
flame ignite quicker because the
particles are closer:
Just before TDC, the spark plugs (in
the cylinder head, near the valves)
ignite the mixture with sparks from a
high-voltage electric current
provided by the magneto, which is
rotating in sympathy with the engine:
It is timed this way to give the fuel
time to catch fire, and produce the
optimum expansion at 10° after
TDC, which is when it is actually
required. The piston is forced
Engines & Systems 51
downwards again, in a smooth
movement, making the crankshaft
rotate, and whatever is attached to it.
The volumetric efficiency is the measure
of mass charge to the theoretical
mass charge at ISA if the engine
were stationary. In other words, the
degree to which the cylinder is filled
with new mixture at full throttle, as
compared to an equivalent amount
of atmosphere. It is rarely more than
80%, due to various leakages and
losses, hence the need for
supercharging, above.
The momentum of the engine
(supplied by the flywheel) brings the
piston up again, to force the exhaust
gases out through the exhaust valve,
which has already opened, just as the
piston hit BDC:
It closes as the piston gets to TDC,
after the fuel valve has opened early,
to allow some atmospheric pressure
in that will help push the exhaust
out, as the piston by this time is
starting to slow down, ready to
reverse direction.
There – that wasn't so bad, was it?
Note that, although there were four
cycles, the crankshaft only went
round twice.
Turbines
As mentioned above, the same
principles apply to jet engines as
reciprocating ones, only they're
applied in a different way. They also
use cheaper fuel, as compression is
not a factor in producing the power,
although avgas can sometimes be
mixed with jet fuel (see the Flight
Manual), at the expense of reduced
maintenance periods, as it doesn’t
the fuel pumps so well.
Turbine engines are discussed in this
book only in relation to helicopters,
because it is more likely that
somebody with a Private Pilot's
Licence is going to fly one.
Because so much air is used for
cooling, humidity has less of an
effect on jet engine performance.
The five basic parts of a jet engine
are the inlet, the compressor, combustor,
turbine and nozzle (the bit joining the
compressor to the combustor is
called the diffuser). They could be
combined or doubled back on
themselves in some engines,
particularly those in helicopters, to
save space, speaking of which,
remember that the engine is not
directly responsible for pushing the
machine forward, as it might be in a
jet aeroplane. Instead, it drives the
main rotor gearbox, which drives the
rotors – the disk formed by them is
what flies and pulls the rest of the
helicopter with it. In such
circumstances, the engine could even
be upside down, provided the
gearing could cope with it. In fact,
the PT6 has a "backwards" design,
with the compressor at the rear.
Another name for a jet engine is a
gas producer because, in a helicopter,
52 JAR Private Pilot Studies
　
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