Plane & Pilot
Tuesday, May 10, 2011

Caring For Your Aircraft

Part III: All About Your Aircraft’s Ignition System

Your starter connects to your propeller through a gear so the propeller turns. This is connected to your engine's crankshaft, so it turns, too. The crankshaft turns a rotor inside the magneto, producing a high-voltage electrical pulse that goes out through the wiring harness and into the spark plug in each cylinder. That spark ignites the fuel-air mixture, and your engine is alive! At this point, the starter and battery can be thrown away, and your engine would keep running just fine (lawn mowers don't have batteries or starters, and they keep running too). Without a starter or battery, hand-propping takes the place of the starter, turning the propeller a little bit to get the magnetos firing. The alternator uses the turning engine to generate electricity, and stores it in the battery to run things like radios and lights, but it has nothing to do with running the engine.

The magneto acts as a generator, providing electric pulses to the spark plugs through the wiring harness.
Once the engine is started, it becomes almost like a perpetual-motion machine; the engine turns the magnetos that produce another spark, which turns the crankshaft, which is attached to the propeller, and it goes on and on, thousands of times per minute until the fuel runs out. Turning your key to "OFF" interrupts the electrical flow through the magnetos, and grounds them through a wire called the "P-lead" (the "P" stands for "primary lead") that runs from the magneto to the ignition switch. That stops the high-voltage pulses from reaching the spark plugs, effectively stopping the engine.

You now can see why your instructor taught you to momentarily turn your key to "OFF" then back "ON" to see if the engine started to die. That's to check the condition of the P-lead. If the engine didn't momentarily hesitate, it would mean the P-lead was faulty and didn't ground (turn off) the magnetos. Thus, even if the key was turned to "OFF" or removed entirely, moving the propeller by hand (such as during a preflight inspection) would cause it to generate an electric pulse from the ungrounded magneto to the spark plug, starting the engine with you standing in the propeller arc.

Star Of The Show
The star of the entire ignition show is the magneto. In addition to lawn mowers and other small motors, magnetos were used in the earliest telephones to generate the ring. They've been around since English physicist Michael Faraday presented his law of electromagnetic induction in 1831. The principle of the magneto goes back to your high-school science class, where you discovered you could create magnetism by wrapping a coil of wire around a piece of iron, and running an electrical current through that coil (from a battery). This was called an electromagnet.

Magnetos work on the opposite principle; a magnet rotated near a coil of wire creates an electrical current. Inside your magneto casing are several components. First is a rotor that turns when your engine's crankshaft turns. A strong magnet is built into that rotor, with a north and south pole (like all magnets). Above the rotor (parallel to the rotor's rotation) is a piece of iron wound with about 250 turns of heavy-gauge wire (the "primary coil"). Following Faraday's law, the rotating magnet creates an electric current (also called "flux") that flows from the north pole of the magnet to the south pole, through that primary coil.

The primary coil is covered with special insulation, followed by about 18,000 turns of a very fine-gauge wire. This "secondary coil's" job is to boost that original electric pulse that went through the primary coil to as much as 20,000 volts. When it reaches its peak strength (and timed perfectly with each cylinder), the breaker points are opened, releasing the high-voltage pulse to the distributor, which sends it down the wiring harness to the spark plug where ignition occurs. In a four-cylinder application, one complete rotation of the rotor sends a pulse to two of the cylinders in your engine, one after the other. It's a beautifully timed and complex, yet simple, dance of electrons.


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