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Chapter 4 — Powerplants
Ignition System
The ignition system provides the spark that ignites the
fuel-air mixture in the cylinders. Components include
a magneto generator, an electronic control box that
replaces mechanical points, spark plugs, high-voltage
leads and the ignition switch(es). Individual manufacturer
designs will vary and pilots must be familiar
with the aircraft operating procedures for the PPC being
flown.
A magneto uses a permanent magnet to generate an
electrical 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
which uses a separate generator in the magneto.
The electricity from the separate ignition coil on the
magneto generator goes into the ignition control box
where the correct voltage is produced and timed to
fire the spark plugs at the proper time. The magneto
also sends a signal to the electric control box to provide
the timing signal to fire the spark plugs.
Most modern PPCs use an electronic timing system
instead of the mechanical points inside the old magnetos
which also housed the points. Capacitor discharge
ignition (CDI) systems are a common example of an
electronic ignition system. Electronic ignition systems
operate without any moving parts to increase
reliability and efficiency. A CDI system begins to fire
when the starter is engaged and the crankshaft begins
to turn. It continues to operate whenever the crankshaft
is rotating.
Most powered parachutes incorporate a dual ignition
system with two individual coil systems in the magneto,
two individual electronic ignition timing systems
(electric box), two separate sets of wires, and two spark plugs per cylinder. Dual ignition systems
increase overall reliability of the engine. Each ignition
system operates independently to fire one of the
two spark plugs. If one ignition system fails, the other
is unaffected. The engine will continue to operate normally,
although you can expect a slight decrease in
engine power.
The operation of the ignition system is controlled in
the cockpit by the ignition switch(es). Since there are
two individual ignition systems, there are normally
two separate ignition toggle switches.
You can identify a malfunctioning ignition system
during the pretakeoff check by observing the decrease
in RPM that occurs when you first turn off one ignition
switch, turn it back on, and then turn off the other.
A noticeable decrease in engine RPM is normal during
this check. If the engine stops running when you
switch to one ignition system or if the RPM drop exceeds
the allowable limit, do not fly the powered parachute
until the problem is corrected. The cause could
be fouled plugs, broken or shorted wires between the
magneto and the plugs, or improperly timed firing of
the plugs because of the control box.
It should be noted that “no drop” in RPM is not normal,
and in that instance, the powered parachute
should not be flown. Following engine shutdown,
keep the ignition switches in the OFF position. Even
with the battery and master switches OFF, the engine
can fire and turn over if you leave an ignition switch
ON and the propeller is moved because the ignition
system requires no outside source of electrical power.
The potential for serious injury in this situation is obvious.
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 corrected, can quickly lead to failure
of the piston, cylinder, or valves. In less severe cases, detonation causes engine overheating, roughness,
or loss of power.
Detonation is characterized by high cylinder head
temperatures, and is most likely to occur when operating
at high power settings. Some common operational
causes of detonation include:
• Using a lower fuel grade than that specified
by the aircraft manufacturer or operating the
engine after it has been sitting for an extended
period; after 3 weeks or as indicated by
your POH you should drain old fuel out and
replenish with fresh fuel.
• Operating the engine at high power settings
with an excessively lean mixture.
• Detonation also can be caused by extended
ground operations.
Detonation may be avoided by following these basic
guidelines during the various phases of ground and
flight operations:
• Make sure the proper grade of fuel is being
used. Drain and refuel if the fuel is old.
• Develop a habit of monitoring the engine
instruments to verify proper operation according
to procedures established by the manufacturer.
Preignition occurs when the fuel-air mixture ignites
prior to the engine’s normal ignition event. Premature
burning is usually caused by a residual hot spot in the
combustion chamber, often created by a small carbon
deposit on a spark plug, a cracked spark plug insulator,
or other damage in the cylinder that causes a part
to heat sufficiently to ignite the fuel-air charge. Preignition
causes the engine to lose power, and produces
high operating temperature. As with detonation, preignition
may also cause severe engine damage, because
the expanding gases exert excessive pressure on
the piston while still on its compression stroke.
Detonation and preignition often occur simultaneously
and one may cause the other. Since either condition
causes high engine temperature accompanied by a
decrease in engine performance, it is often difficult to
distinguish between the two. Using the recommended
grade of fuel and operating the engine within its proper
temperature and RPM ranges reduce the chance of
detonation or preignition.
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