Ignition System
How does the ignition system function?
The
ignition system is the starting system for your small engine. Whether you start the engine with a tug on the rewind rope or the turn of a key on an electric starter motor, you're relying on the ignition system to produce a spark inside the combustion chamber.
The ignition system includes magnets mounted in the surface of the flywheel, and an ignition armature mounted adjacent to the flywheel, containing copper wire windings. It also includes the spark plug lead (attached to the armature) and the spark plug.
When you pull on the rewind rope, you are turning the flywheel, a heavy metal wheel located under the blower housing. With each turn, the magnets mounted in the surface of the flywheel pass the ignition armature, inducing electrical flow that produces a high-voltage spark at the tip of the lug. The ignition system is coordinated with the timing of the piston and motion of the valves so that the spark will ignite the air-fuel mixture in the combustion chamber just as the piston reaches the point of maximum compression in each engine cycle.
Once the engine is running, the flywheel's inertia keeps the crankshaft spinning until the pistons next power stroke, while the flywheel magnets induce voltage in the armature to keep the spark plug firing.
If you are experiencing ignition timing issues, this is most often due to a sheared flywheel key. For information regarding this, please visit our Inspecting the Flywheel and Key FAQ.
Solid-State Ignition Systems
It takes 10,000 to 20,000 volts of current to produce a spark at the tip of a spark plug. That's enough to give a person a powerful jolt. Today's ignition systems accomplish this using a tiny transistor in the ignition armature. Each time the magnets approach, the transistor establishes an electrical circuit, also called "closing" a circuit. The 2 to 3 amps of current produced are then converted to high-voltage current that travels through the spark plug lead to the spark plug.
Breaker Point Ignition Systems
Breaker point systems are found on small engines built until the early 1980s. They function much like solid-state ignitions, but use a mechanical switch, instead of a transistor, to close the electrical circuit required to produce a high-voltage spark at the spark plug tip. A pair of nickel-plated breaker points remain apart for most of the four-stroke cycle. A flat spot machined into the crankshaft causes one of the points to pivot temporarily, closing the gap between the two and closing a circuit.
For additional information, please visit our Ignition System Theory and Testing page.
Please read and abide by any applicable Safety Information contained in your engine Operator's Manual. The material provided above is not intended to replace work performed by a Briggs & Stratton Authorized Dealer. Terms and Conditions apply to all of the information presented on this website. Always be sure to completely read and understand your engine Operator's Manual.
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