曝光台 注意防骗
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injection, where the fuel is metred
directly to the cylinders according to
power requirements, automatically
taking air density into account. Ice is
not formed because there is no
venturi to cause temperature drops
(there's no carburettor in the first
place, as it is replaced with a fuel
control unit). In fact, the only control
you have (apart from the throttle) is
Idle Cut Off (ICO). The biggest
problem with fuel injected engines is
blocked jets, from dirt in the fuel.
A lesser one, but still significant, is
difficulty in starting, particularly on a
hot day, where the feed pipes lie
across the top of the engine and
consequently get warm (like the PA
31), with the fuel inside them
evaporating nicely and creating a
vapour lock, so you need a short burst
of fuel pressure from the pumps to
prime the lines. If the primary airway
is blocked (ice, or a bird), an alternate
air switch in the cabin will 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
206 Canadian Professional Pilot Studies
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 course by a
wastegate. When the wastegate is
closed, the engine is 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.
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 with 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.
Airframes, Engines & Systems 207
In a rotating magnet magneto, a rotor
rotates in the gap between two ends
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