Ignition Systems Flashcards
Electric starter motor
Powered by the battery, drives the engine via a gearbox or a toothed flywheel
Overrun clutch disengages the drive from the motor
What illuminates whilst the drive gear is engaged and a possible indication of a problem
An amber start warning light illuminates
If on for 30 secs, the motor may have failed - immediately shut down engine
The start process of a piston engine
Electric start motor drives the engine
Engine rotation drives the magnetos which deliver high tension electricity to the spark plug
Must provide a spark at precisely the right time in the cycle to ignite the mixture
How does it control the ignition system
Each magneto is controlled by its own ignition switch
The switch isolates the ignition circuit by connecting the primary circuit to the earth (Grounded cndtion)
Switch closed = earth, open = live
Provides fail safe operation in flight but on the ground a disconnected wire = permanently live circuit
Advantages of using 2 spark plugs
Helps accelerate the combustion process as the flame spreads from 2 points simultaneously
Quicker rise to peak cylinder pressure and a noticeable increase in power - decreases when you switch the magneto off
Why are magnetos pressurised
Reduces the possibility of internal arcing t altitude
A magnetos output voltage is directly proportional to
Engine RPM
Purpose of spark augmentation and different types
Spark occurs too early/advanced at low RPM so the start procedure needs help
Impulse coupling
LT and HT boosters
Impulse coupling
Occurs every half a revolution, ensures sufficient strong sparks and retards the spark
Engine turns the drive spindle - winds up a spring loaded clutch - when the spring releases it rapidly spins and spins the magneto
HT/LT boosters
Uses a battery to get a sufficient spark and to retard the spark
LT = Primary
HT = Secondary
How many volts supply a spark plug
30,000 volts
1st mag check
Just after start at 1000 - 1200 RPM
Each magneto is turned off in turn
RPM should drop but the engine should continue to work smoothly
Proves the operation of the switches and 1 magneto is enough
2nd mag check
At around 75% RPM
Each switched off in turn
RPM drop must be within limits
Success = if one fails the power should not be effected
Fouling
Oil,carbon and lead deposits can block the gap for the spark
More likely when operating outside the specified temp range
Magneto dead cut
Indicates a failure of 1 half of the ignition circuit
