Piston Engine
Ignition System
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The Ignition System
Purpose
To provide a spark discharge at the spark plug at the
correct time
There is no electrical connection between the
aircraft electrical system and the ignition system
Failure of the electrical system will not affect the
ignition system
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The Ignition System
The source of electricity to create a spark in a car is
the battery, therefore if a car’s electrical system fails
so does the ignition system
The source of electricity to create a spark in an
aircraft electrical system is the magneto, a self
container generator totally independent of the
electrical system
The primary aim of using a magneto instead of
battery ignition is the safety provided by its
independence.
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Battery Ignition System
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Magneto Ignition System
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Magneto Ignition System
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Magneto Ignition System
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The Primary Electrical Circuit
The primary electrical circuit consists of a set of: breaker contact
points, a condenser, and an insulated coil.
The coil is made up of a few turns of heavy copper wire, one end
is grounded to the coil core and the other end to the ungrounded
side of the breaker points.
The condenser (capacitor), is wired in parallel with the breaker
points to prevents arcing.
The breaker points close and open in the same relation to each of
the four neutral positions of the rotor magnet.
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The Secondary Electrical Circuit
The secondary circuit contains the secondary windings of the coil,
distributor rotor, distributor cap, ignition lead, and spark plug.
One end of which is electrically grounded to the primary coil or to
the coil core and the other end connected to the distributor rotor.
The secondary coil is made up of a winding containing approximately
13,000 turns of fine.
The primary and secondary coils are encased in a non conducting
material. The whole assembly is then fastened to the pole shoes with
screws and clamps.
A very high-voltage, often as high as 20,000 volts, is generated in the
secondary circuit
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Magneto
Ignition
System
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The Distributor
The distributor consists of a rotor turning at half engine speed (on
a 4-stroke engine) inside a circular cover. The cover has as many
brass segments symmetrically placed round the inside of the cover
as there are cylinders. The current from the secondary winding is
fed to the rotor and in turn to the appropriate brass segment to feed
the sparking plug. The firing order to the cylinders is arranged
to even the load along the crankshaft. For instance on a 4 cylinder
engine, the firing order is usually, starting from the front cylinder,
1, 3, 4, 2.
The firing order of a nine-cylinder radial engine is 1-3-5-7-9-2-4-
6-8
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Spark Plugs
The function of the spark
plug in an ignition system is
to conduct a short impulse of
high-voltage current through
the wall of the combustion
chamber.
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The Ignition System
The ignition system provides the spark that
ignites the fuel/air mixture in the cylinders and
is made up of:
• Magnetos
• Spark plugs
• High-tension leads
• Ignition switch
• Breaker points for spark timing
• Distributor: to direct the magneto electrical
energy to the correct spark plug
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The Ignition System
A magneto is simply an electrical generator
consisting of a coil and a magnet
All piston aircraft engines have dual magnetos
for safety should one fail.
The magneto is driven by the crankshaft, so once the
engine is running the magneto is self sustaining.
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The Ignition System
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Low-Tension Magneto System
High-tension ignition systems have undergone many
refinements and improvements in design. This includes new
electronic systems that control more than just providing
ignition to the cylinders. High-tension voltage presents
certain problems with carrying the high-voltage from the
magneto internally and externally to the spark plugs.
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The Ignition System
For a spark to occur, very high voltage is required.
This is normally in excess of 20,000 volts
At these high voltages leakage from the high
tension leads can occur, especially at high altitudes.
As the air becomes less dense, its insulating
capability decreases.
The high voltages that pulsate along the leads emit
radio frequency which will interfere with VHF radio
operation
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The Ignition System
To prevent this from occurring the ignition
harnesses are covered with a protective coating
Occasionally the protective coating cracks. We can
tell by the RF noise generated on the VHF radio.
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FADEC System Description
A FADEC (Full Authority Digital Engine Control)is a solid-state
digital electronic ignition and electronic sequential port fuel
injection system with only one moving part that consists of the
opening and closing of the fuel injector. FADEC continuously
monitors and controls ignition, timing, and fuel
mixture/delivery/injection, and spark ignition as an integrated
control system.
FADEC monitors engine operating conditions (crankshaft speed,
top dead centre position, the induction manifold pressure, and
the induction air temperature) and then automatically adjusts
the fuel-to-air ratio mixture and ignition timing accordingly
for any given power setting to attain optimum engine
performance.
As a result, engines equipped with FADEC require neither
magnetos nor manual mixture control.
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The Starter Relay
Relays and solenoids are used when it is necessary to open
or close a circuit carrying a large amount of current from
a remote location and with a small switch
This component of the ignition system wouldn’t be
necessary if you were happy hand swinging the prop
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Ignition Switch
• Controls the operation of the magneto from the
cockpit
• Can select left, right or both
• Engine can still fire even with the battery and master
switches off, if the magnetos are left on and the prop
is moved because no outside electrical power is
required.
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Key on Left Magneto
L Magneto L Spark Plug
R Magneto R Spark Plug
Turning the key to the left
magneto position earths the
right magneto
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Key on Right Magneto
L Magneto L Spark Plug
R Magneto R Spark Plug
Turning the key to the
right magneto position
earths the left magneto
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Key on Both Magnetos
L Magneto L Spark Plug
R Magneto R Spark Plug
With the key in the BOTH
position, neither magneto
is earthed
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Key on Both Magnetos
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Key on OFF position
L Magneto L Spark Plug
R Magneto R Spark Plug
With the key in the OFF
position, both magnetos
are earthed
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The Impulse Coupling
Many opposed reciprocating engines are equipped with
an impulse coupling as the auxiliary starting system. An
impulse coupling gives one of the magnetos attached to
the engine, generally the left, a brief acceleration, that
produces an intense spark for starting. This device
consists of a cam and flyweight assembly, spring, and a
body assembly.
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The Impulse Coupling
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The Impulse Coupling
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An aircraft electrical system operates at approximately
constant
A. voltage
B. amperage
C. resistance
And….. the answer is………..
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If the switch wire leading from the impulse
magneto to the cockpit key was severed, the effect
on the engine would be
A. the engine would not start as it is affecting
the impulse magneto
B. the engine will continue to run if both magnetos
are turned off
C. the impulse mechanism will be in-operative
D. the engine will stop when the other magneto is
switched off
And….. the answer is………..
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If the alternator failed in flight the outcome would be
A. all electrical instruments would immediately fail
B. the engine would continue to run
C. the engine would stop when the battery went flat
D. the engine would immediately stop
And….. the answer is………..
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It is advisable to check both magnetos to make sure they
are capable of being switched off. This check should be
made
A. during the 1700 RPM engine run-up prior to
flight
B. before shutting the engine off after flight
C. before starting the engine
And….. the answer is………..
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The earth wire coming from the magneto is
connected to
A. the nearest metal part of the aircraft
B. the case of the magneto
C. the magneto key switch
D. any metal part of the aircraft
And….. the answer is………..
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The capacity of a battery is measured in
A. amps
B. amp hours
C. volt hours
D. volts
And….. the answer is………..
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