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vapour lock, so you need a short burst
of fuel pressure from the pumps to
prime the lines.
If the primary airway is blocked
(maybe from ice, or a bird), there
will be an alternate air switch in the
cabin to change the source.
An electric fuel injection system will
be backed up by a mechanical one.
The benefits include being able to
use it without the engine running,
making it useful for priming.
Superchargers
A supercharger is a compressor run
directly by the engine, situated
between the carburettor and the inlet
manifold. Its function is to extend the
service ceiling of the aircraft, by
compressing the fuel/air mixture to
maintain sea level power at altitude,
or to increase normal power lower
down. This forced induction only really
works in a particular temperature
range. The essential point to
remember is that the extra air is
sucked through the carburettor and
blown into the cylinder.
Turbochargers
A turbocharger, on the other hand,
is powered by exhaust gases which
are deflected from their normal
course outwards by a wastegate. When
the wastegate is closed, the engine is
being turbocharged. The gases drive
an impeller that also compresses air,
but at the intake (before the carb, so
the air is being blown into it). Also, the
engine doesn't lose power by driving
it. Since the exhaust system is
involved, preflight checks should
include security of the pipes, so
carbon monoxide doesn't get into
the cabin.
Automatic waste gate control is done
with engine oil pressure acting
against a spring, which opens the
gate at low altitude. Oil closes it at
high altitude, although, at low engine
power it will be closed anyway, to
conserve pressure. The critical altitude
is the pressure altitude at which it is
fully closed. Above it, power will fall
in line with the manifold pressure.
The waste gate actuator controller is
downstream of the actuator.
Turbocharger boostrapping (or hunting) is
an overreaction to rapid throttle
movement, which leads to large
pressure fluctuations and possible
overboosting. A pressure relief valve
in the induction manifold cures it.
The turbocharger controller can also
be damped.
Turbocharger intercoolers (between the
compressor and throttle) are there to
prevent detonation at high altitude.
The turbocharger is lubricated with
engine oil.
Airframes, Engines & Systems 49
Ignition
This is the whole mechanism that
provides the spark at the critical
moment, consisting of plugs, leads,
magnetos, switches, etc., in duplicate
(one magneto will serve one plug per
cylinder, and the second the others).
The duplication is actually for
efficiency, as the magneto doesn't
work that well at low RPM, but a
side benefit is, obviously, safety
(watch the RPM drop as you switch
to single mags in the runup to see
what I mean about efficiency).
The magneto is a device that has a
transformer and all the circuitry to
boost the low voltage primary
current to one large enough to jump
across a small gap at the plug
electrodes (around 15,000 volts). A
car has similar items, but not in one
unit. The difference is that a
magneto works as long as the engine
is turning it – there is otherwise no
need for external influence.
According to the Electricity & Radio
chapter, this is done with the help of
magnetic induction, or the relative
movement of a magnetic field
around a conductor, in which a
current is induced. On the other
hand, if a current flows in a
conductor, an associated magnetic
field varies with the current flow.
In a rotating magnet magneto, a rotor
rotates in the gap between two ends
of something like a horseshoe
magnet, called an armature, the bulk
of which is surrounded by two sets
of coils, a primary and a secondary
winding. As the rotor spins, the flux
along the armature reverses as the
lobes pass the ends (in the above
case, you will get four flux reversals
and four sparks per revolution, so it
runs at ¼ of the engine speed).
The primary coil oscillates in sympathy
with the flux reversals, and a current
is induced in the secondary coil
wrapped around it (because the
primary field is expanding and
collapsing – the essential point is
movement). The ratio between the
coils is significant, which is how
such a high voltage is generated. The
primary has thicker wire and
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