Gasoline-powered lawn and garden accessories use a spark plug but these small engines do not have a battery on board. Push lawn mowers, edging tools, hedge trimmers, chain saws and blowers all have an engine. So how does the spark plug get its fire? Instead of relying on a battery for the burst of energy, these engines use a magneto to generate the power. Because magnetos are extremely reliable and don't have to be repeatedly charged, they are among the most popular ignition devices. Some gasoline-powered propeller driven airplanes also use magnetos.
To crank an engine, the ignition system has to generate an extremely high voltage at exactly the right moment. To ignite the fuel in the motor, this initial burst of energy needs to be in the neighborhood of 20K volts. The sudden burst of voltage causes the electrical pulse of energy to jump across a spark plug's gap igniting the air/fuel mixture in the combustion chamber of the cylinder. Since this action only requires the electrical generating device to provide a periodic pulse of high-voltage, a magneto is one of the simplest solutions for delivering the spark.
For the most part, a magneto is the reverse action of an electromagnet. When you apply electrical current to an electromagnet, the coil of wire around the armature (iron bar) creates a magnetic field. If you reverse the process, you generate electrical current by moving a magnet past the armature. The magnetic field induces a small amount of current in the windings of the primary and secondary coils. When the maximum amount of energy is produced, an electronic control unit (a set of breaker points and a capacitor) opens. The primary coil produces a small voltage that is amplified hundreds of times by the secondary coil.
If you have a riding lawnmower with a starter or headlights for mowing in the dark, you've likely discovered the battery box under the seat. However, that does not mean your spark plug receives it's boost from the battery. Many smaller riding mowers continue to use the reliable magneto for that big burst of power sent to your E3 lawn and garden spark plug.
