A Typical Ignitor and How It Works
An ignitor, used on some of these early engines, was the early forerunner of todays' spark plug, and provided ignition for these engines by mechanical as well as electrical means. Unlike a spark plug, the voltage used is very low, only a few volts. The electrical current is the key to this type of ignition system.
As seen in the above picture, a typical ignitor is nothing more than a set of electrical contacts, a switch if you will, that are mounted inside a cavity which is part of the combustion chamber of the engine. These contacts are opened and closed via a shaft to the outside of the engine where a snap action mechanism is tripped, usually, by part of the exhaust valve train.
The electrical circuit of the ignitor system consists of a battery(s), a simple coil and the ignitor itself, or simply a low voltage magneto along with the ignitor. As seen above, the ignitor stationary electrode is insulated from the body of the ignitor by a set of Mica washers and a Mica or non-conductive tube. The electric source is attached to this electrode via a small clip on the outside of the ignitor. The movable electrode is not insulated and is electrically common to the engine block which is the return path for the electrical source. A simple inductive coil is used in series with the battery and ignitor to provide the extra energy to cause a bright blue spark between the ignitor contacts when they are opened. The mechanism located on the outside of the ignitor, along with the exhaust cam and push rod, causes the contacts to snap open just prior to the piston reaching top dead center (TDC) on the compression cycle, thus igniting the air/fuel charge in the combustion chamber.
Mechanically, a set of small iron castings, the 'hammer' and the 'anvil' are mounted on the movable contact shaft. A small steel finger or 'trip arm' usually mounted to the push rod, comes in contact with the end of the ignitor 'hammer' prior to point of ignition and slightly rotates the hammer on the shaft. This winds the torsion spring, which itself is fixed to the shaft by a cotter pin. Torsion or twisting of this spring overcomes the 'opening' tension spring, and turns the shaft slightly, closes the contacts, and completes the electrical circuit. As the trip arm continues to move into the hammer, the torsion spring continues to wind up. When properly adjusted, the trip arm will slip off the hammer at the precise time ignition is required. The combined strength of the tension and torsion springs snap the hammer back to its orignal position. The hammer will strike the anvil which is clamped tightly to the movable shaft. This causes the shaft to snap back to its original position, snapping the contacts open and creating the spark.