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LSA engines designed specifi cally for aircraft, a magneto/
generator uses a permanent magnet to generate an electric
current independent of the aircraft’s electrical system, which
might include a battery. The aircraft electrical system can
fail—the battery can go dead. However, this has no effect
on the ignition system.
The electricity from the ignition magneto/generator goes into
the ignition control box where the correct voltage is produced
and timed to fi re the spark plugs at the proper time. Modern
WSC aircraft use an electronic capacitance discharge system
that operates without any moving parts to increase reliability
and effi ciency. Capacitance Digital Systems (CDI) operate
similarly but they have the ability to change the timing of the
spark for different rpm. Consult the POH for the particular
system for each engine.
The system begins to fi re when the starter is engaged and the
crankshaft begins to turn. It continues to operate whenever
the crankshaft is rotating. Most WSC aircraft incorporate
a dual ignition system with two individual magneto/
generators, separate sets of wires, separate sets of control
boxes, and separate sets of spark plugs to increase reliability
of the ignition system. Each magneto/generator operates
independently to fi re one of the two spark plugs in each
cylinder. If one of the systems fails, the other is unaffected.
The engine will continue to operate normally, although a slight
decrease in engine power can be expected.
The operation of the magneto/generator output to the ignition
system is controlled in the fl ight deck by the ignition switch.
Since there are two individual ignition systems, there are
normally two separate ignition toggle switches or separate
positions on the ignition control, as shown in Figure 4-16.
Identifi cation of a malfunctioning ignition system during
the pretakeoff check is observed by the decrease in rpm that
occurs when fi rst turning off one ignition switch, turning it
back on, and then turning off the other. A noticeable decrease
in engine rpm is normal during this check. If the engine
stops running when switching to one ignition system or if
the rpm drop exceeds the allowable limit, do not fl y until
the problem is corrected. The cause could be fouled plugs,
broken or shorted wires between the magneto/generator and
spark plugs, or improperly timed fi ring of the plugs because
of a defective control box. It should be noted that “no drop”
in rpm is not normal, and in that instance, the aircraft should
not be fl own. Following engine shutdown, keep the ignition
switches in the OFF position. Even with the battery and
master switches OFF, the engine can fi re and turn over if
an ignition switch is left ON and the propeller is moved
because the magneto/generator requires no outside source
of electrical power. The potential for serious injury in this
situation is obvious.
Standard category aircraft engine systems are described in
the Pilots Handbook of Aeronautical Knowledge; however,
these engines are not typically used on WSC. Automobile
engines or other non aircraft engines may be used on WSC
where the ignition system runs off the battery rather than a
magneto/generator system. In this case if the battery system
fails, the engine ignition system will fail and the engine will
stop.
Combustion
During normal combustion, the fuel/air mixture burns in
a very controlled and predictable manner. Although the
process occurs in a fraction of a second, the mixture actually
begins to burn at the point where it is ignited by the spark
plugs, then burns away from the plugs until it is consumed
completely. This type of combustion causes a smooth buildup
of temperature and pressure and ensures that the expanding
gases deliver the maximum force to the piston at exactly the
right time in the power stroke.
Detonation is an uncontrolled, explosive ignition of the
fuel/air mixture within the cylinder’s combustion chamber.
It causes excessive temperatures and pressures which, if not
4-13
Fuel tank
PRIMER
Fuel pickup with screen
Carburetor fuel bowl
Optional primer plunger
Fuel filter
Optional primer bulb
used to pump fuel
into the carburetor
fuel bowls for starting
(an alternative electric
fuel pump can also
be used)
Fuel fill with
integral vent
Throttle Control
Engine driven
fuel pump
Figure 4-17. Typical Carburetor Fuel System.
corrected, can quickly lead to failure of the piston, cylinder,
or valves. In less severe cases, detonation causes engine
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Weight-Shift Control Aircraft Flying Handbook(45)
