Ignition System

Question 1

Purpose of ignition system

Answer 1

Ignite the fuel/air mixture at the proper time.

Question 2

Purpose of magnetos

Answer 2

Produces electricity for spark plugs
Boosts voltage to approximately 20,000 volts
Distributes electricity to each spark plug at the proper time

Question 3

Dual magneto ignition system

Answer 3

Each magneto has its own distributor, wires, and spark plugs
Each cylinder has two spark plugs, one for each magneto
Two independent ignition systems that are independent of the battery/alternator system
Redundancy ensures safety
Provides more complete combustion & even fuel burn

Question 4

Magneto theory

Answer 4

Gear-driven by the engine
Rotating magnets develop and distribute electricity through coils, a condenser, and breaker points to the spark plugs that ignite the fuel/air mixture
High voltage for spark plugs comes from voltage step-up in the magneto coil

Question 5

Ignition timing

Answer 5

Timing of the ignition and the valve events
Aircraft engine magnetos have adjustable timing (by the mechanic)
Timing is adjusted separately on each magneto so that it fires at exactly the right time
Engine malfunctions are symptoms that timing may not be right
Normal ignition occurs as piston is ending compression stroke (advanced)
Ignition spark is mechanically retarded during engine start
●Occurs later (retarded) than normal advance position
●Piston at or near top dead center
●Prevents engine kick-back

Question 6

Impulse coupling

Answer 6

•As starter rotates engine, flyweights and a spring produce a hot, late spark
•Once engine fires, flyweights retract for normal ignition

Question 7

Primary lead

Answer 7

•The P-lead grounds the magneto and interrupts current flow to the spark plugs
•When the magneto/ignition switch is OFF
•A short circuit (grounding) in the magneto coil prevents it from working
•A path to ground exists and the magnetos will not spark the plugs
•The wire that does the grounding is the
P-lead
•OFF means both mags are grounded
•Switching to the LEFT (L) mag grounds the right mag (shuts it off)
•BOTH position removes the ground from both mags & the engine uses the full dual system
•A defective P-lead could cause an engine start if the propeller is moved (“hot-prop”)

Question 8

Engine run up

Answer 8

•Magneto/ignition switch check
•1700-2000 RPMs
•Tolerances
•150-175 RPM maximum drop
•50 RPM maximum difference
•Excessive drop may indicate several problems including fouled plugs, bad ignition wire, improper timing, or bad magneto