How Does a Briggs & Stratton Electric Starter Work?

Starter Housing

• The starter motor housing is primarily a short, squat tube. Magnets are affixed to the inside wall of the housing, and the starter shaft is suspended in between the magnets. The starter shaft is held in place by bearings in the end plates of the housing. One end of the shaft reaches through the housing end cap nearest the engine flywheel. This end of the starter shaft engages the flywheel and turns the engine over.

The Shaft

• Inside the housing, the starter shaft is wound with wires and is called the armature. The armature is energized when the ignition switch is turned to the "start" position, much like in a car. A 12-volt lead-acid battery provides the power and this energy disrupts the magnetic fields. The resulting reaction spins the armature. As the armature spins it maintains electrical connection through contacts, or "brushes," that ride on a portion of the armature.

The Starter Drive

• As the starter shaft spins the drive gear engages the flywheel ring gear, and turns the engine. Once the engine starts, the flywheel ring gear speed quickly surpasses the starter drive gear speed. A one-way clutch, or "sprag," allows the drive gear to free-wheel when spun by the engine ring gear. Surface tensions relax, and the drive gear retracts with the aid of a return spring.

Engagements

• Two methods are used to make the starter drive gear engage the engine flywheel gear. Centrifugal force created by the starter shaft turning speed is used to "sling" the gear forward against return spring pressure. The torque produced keeps the gears engaged until the engine speed overtakes the starter gear speed. Solenoids are used to accomplish gear engagement through electro-magnetic induction. Mechanical action created by energizing the solenoid forces the starter drive gear forward.