- The primary coil in a small gas engine is part of the ignition system. The ignition system provides the electricity to make the spark that ignites the fuel-air mix. This type of ignition system is called a magneto ignition and can produce as many as 20,000 volts.
- A primary coil consists of a metal frame that has two or three legs, making it shaped like an inverted U or E, around which is wound a few hundred turns of wire. This wire is called the primary winding. A second winding that has thousands of turns of very fine wire is wrapped around the smaller coil. This coil is called the secondary winding. The combination of the two coils of wire, one much larger than the other, forms a step-up transformer that multiplies any voltage in the primary winding by many times.
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The primary coil is mounted with the open legs of the frame only a few thousandths of an inch away from the engine flywheel. The flywheel is a heavy disk of metal that has two magnets embedded in its surface on one side. The poles of these magnets are mounted opposite each other. As the flywheel turns the magnets pass under the ends of the metal frame of the primary coil. The passing magnetic field induces a voltage in the primary coil. The spinning magnets rapidly pass by the coil, reversing and then removing the magnetic field.
At the same moment the electronic ignition module breaks the current in the primary winding, voltage is induced in the secondary winding, which steps up the voltage in the primary coil. This forces a spark to jump across the gap in the electrode of the spark plug, igniting the fuel-air mix in the cylinder. Older engines do not have an electronic control module and instead use a set of ignition points and a condenser to break and store the primary voltage.
