How It Works: Magneto

Find out what's going on behind the fire wall to excite those spark plugs.

A quick turn of the key is all a pilot usually needs to start the engine and go, but have you ever wondered what’s really going on beyond the fire wall to excite those spark plugs? The aircraft components that make it happen, of course, are the magnetos — simple and highly reliable electrical generators. Here’s how magnetos work:

REDUNDANCY Airplanes have two magnetos, left and right, each of which fires one spark plug per cylinder, creating a redundant system that allows the engine to operate at full power independent of the engine-driven alternator.

VOLTAGE CREATION Each magneto features a permanent magnet on a rotor that spins in close proximity to a high-output coil of wire with two windings: a primary winding made of heavy copper wire and a secondary winding of fine wire with an exponentially higher number of turns. The magnetic flux lines that pass through the turns of the primary winding create what are called "magnetic flux linkages." As the magnet moves, the number of magnetic flux linkages changes, creating voltage.

ELECTRICAL SPIKE Because this voltage is not nearly strong enough to fire the spark plugs, the system must rely on the secondary winding to create more ­electricity, about 100 times that of the primary winding. This voltage is created on the secondary winding when the breaker points connected to the primary winding open, causing the magnetic field in the primary coil to collapse and then prompting a large change in magnetic flux linkages. This spike in electricity crosses over to the secondary winding, which greatly amplifies it, creating anywhere from 20,000 to 30,000 volts. As the secondary coil releases its high voltage, the spark plugs ignite the fuel-air mixture in the cylinder.