IGNITION SYSTEM

A close look at how the ignition system works will demonstrate the importance of the flywheel magnets. They are used in the conventional magneto ignition system and in the typical new transistorized ignition systems used on a few premium mowers and blowers.
The conventional system features a coil, called a magneto, mounted on the engine very close to the flywheel. Like an automotive ignition coil, ithas two windings, one of relatively thick wire, another of many more turns of thinner wire. See 1-13. The thicker wire is connected to a set of breaker points and a condenser, just as in older automobiles, as shown in 1-14 and 14a. The breaker points are simply two electrical contacts-a switch. One is fixed, the other is movable. A lobe, on the crankshaft, somewhat similar to those on the camshaft that operate the valves, pushes open the points once every crankshaft revolution. When the lobe spins away from the movable point, a spring pulls it back into contact with the fixed point. The points are wired into an electrical shock absorber called a condenser, which absorbs stray high voltage during the firing of the spark plug, to prevent premature burning of the points. When the points are closed, they complete a circuit to electrical ground. When they are opened by the crankshaft lobe, they interrupt the circuit. Here’s how they are used to create high-voltage electricity that ignites the air-fuel mixture in the cylinders: As the flywheel spins, in I-14 and I4a, the magnets pass the coil, which converts the magnetism to electrical energy. The electricity flows through the thick wiring of the coil to the ignition breaker points, through them (when they are closed) to electrical ground, completing what is called the primary circuit. When the breaker points open, as in 1-15 and 15a, the interruption in the circuit causes the primary circuit to collapse. The electricity does not disappear, but is transferred to the thin-wire winding. Because the second winding is thinner, the same amount of current that flowed under low pressure through the thick winding must now be compressed in the thin winding. Electrical pressure is called voltage, and what happens is that the voltage increases tremendously in rough proportion to the number of wire windings in the primary circuit versus those in the secondary. A typical magneto coil may have 150 turns of primary winding and 10,000 turns of secondary, ratio of 1 to 70. If primary voltage were 300-400 volts, the secondary might be as high as 20,000. The principle of forcing a low voltage circuit to collapse on one of many more windings of wire to produce high voltage is basically what is applied in electrical utility transformers. The reason that the current transfers from the thick wire to the thin wiring is that electricity takes the path of least resistance to find a way to complete a circuit. The ignition system is designed so that this electricity will complete a circuit through the spark plug. When the high voltage forms in the secondary, it looks for the easy way out. Some of it tries to jump across the breaker points, but the condenser temporarily absorbs it. The rest travels along a wire to the spark plug, which has two tips that are positioned in the combustion chamber. These tips are separated from each other, but one is part of the plug’s metal shell. High-voltage electricity travels down to the core tip, thenjmnps a small gap (perhaps ,025 inch) to the other tip. The jumping of the electricity from one tip to the other, through an air gap, is ‘what we call a spark, and it ignites the air-fuel mixture.
When the current reaches the second tip, it has completed its journey, because the plug is threaded into the cylinder head and electrical ground.