Video: how a pre-engaged starter motor works
The pinion on a pre-engaged starter motor fully engages with the ring-gear before rotating at high-speed. Unlike inertia starter motors, there is no collision between a rapidly rotating pinion and a stationary ring-gear.
The pre-engaged starter motor is fitted with a relay valve which controls the movement of the pinion. When the starter motor is activated the pinion moves along the shaft until it comes into contact with the toothed ring-gear.
The pinion interlocks with the ring-gear before rotating to crank the engine.
As the engine gets up to speed the pre-engaged starter motor is protected from overspeed by a free wheel clutch. At this stage the return spring retracts the pinion to its disengaged position.
Watch this slow-motion video to see the pre-engaged mechanism in action:
Only truly pre-engaged starter motors can confidently claim to stop sparks being produced during the cranking process. Other technologies – sometimes called “soft start” – are usually inertia starters that rotate at less than full speed when engaging the pinion.