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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.
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
generates about 50 volts, but the
secondary wire is hair-thin and
produces over 15,000.
Inside the primary circuit is a set of
contact points that act as a circuit breaker
whose function is to make the flux
reversals more abrupt, since they are
not good enough to produce a clean
signal by themselves. In short, the
points are there to make and break
the primary circuit.
There is a cam which moves a rocker
arm to the open position as many
times per revolution as there are
lobes on the cam. There is a spring
to return the rocker arm to its
normal position. The separation of
the points needs to take place in
strict timing with the maximum
primary current, so the field
collapses at maximum intensity.
As the points separate, a spark is
generated across them as the primary
current ceases, due to a strong
follow-on current in the primary
(electricity has a momentum). A
condenser is used in parallel with the
points to minimise the spark, but the
points still need to be looked at for
pitting and damage because it can
never be eliminated entirely.
50 JAR Private Pilot Studies
The high-tension current generated
is fed to the plugs by a distributor,
which is essentially a rotor spinning
inside a cap holding heavy cables
going to the plugs (part of the same
unit as a magneto, but separate in a
car). The one in the middle is the
King Lead. When fitted, they are not
matched to the plugs in order, as one
cylinder would receive the spark at
　
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