Briggs & Stratton developed the Magnetron ignition system in the 1980s. Similar to previously induction ignition systems, the Magnetron system doesn’t rely on mechanical factors to create current to the spark plug. Rather, the switching is performed by means of a set of transistors. Magnetron ignition systems are so reliable that they have replaced breaker point systems in all lawn (Salt Lake City, UT) mower engines produced by Briggs & Stratton as well as in those produced by other manufacturers.
Electro-Magnetic Induction
The ignition system begins a lawnmower and keeps it running by creating enough electric current to leap the gap between the electrodes of a spark plug, and also to make a spark to ignite fuel in the combustion chamber. To do so, the system makes use of the phenomenon of magnetic induction, discovered by physicist Michael Faraday in 1831. Faraday found that magnetism and power are associated, and that moving a magnet past a coil of conducting wire creates electric current in the wire. A corollary is that passing electric current through the coil creates a magnetic field which could induce current in another coil.
Increasing Voltage
A lawnmower engine using a Magnetron ignition system has a flywheel, which is connected to the engine crankshaft. A single permanent magnet is connected to the edge of the flywheel, and also every time the flywheel revolves, the magnet passes a coil of copper wire and also creates an electric current in it. By itself, this current is not strong enough to leap the gap across the spark plug electrodes, however, so the ignition system includes a transformer, which is a secondary coil with more turns than the original one. According to a law which Faraday found, the voltage induced in the second coil by the electromagnetic field generated by the first coil rises with the number of turns in the second coil.
Transistor Switching System
In order to keep the plug from sparking continuously and disturbing piston cycle, then the ignition system must include a switching mechanism to cut power to the plug. In systems which preceded the Magnetron system, the switch was mechanical, typically comprising a spinning breaker which made momentary contact with an electrode to shut the circuit. In a Magnetron system, the switching is executed by a Darlington transistor, which is in fact a set of transistors. When the first of the transistors finds a current from the activate winding, it opens the next transistor to let it pass current to the main coil.
Operation of a Magnetron Ignition
Starting a lawn (Salt Lake City, UT) mower with a Magnetron ignition consists of spinning the flywheel, either electronically or manually, by way of a pull-rope. The voltage induced with each twist goes into the main coil, which has about 75 turns. It induces a bigger voltage in the secondary coil, which has over 4,000 turns, and produces a current of about 10,000 amps, which is sufficient to leap the gap between the spark plug electrodes. The spark ignites the fuel, and also the movement of the piston pushes the flywheel through the other revolution, creating a self-sustaining cycle which forces the lawn in Salt Lake City mower.