Summary
Highlights
An engine starter motor is a powerful electric motor designed to crank an engine. It features a pinion that meshes with the flywheel’s teeth, turning the crankshaft. Modern vehicles commonly use a pre-engaged type starter motor.
The starter motor draws power from the battery. A solenoid, an electromagnet, is connected via an ignition switch. When the ignition switch is turned on, the solenoid energizes, causing a plunger to slide and connect terminals of a contactor switch, completing the circuit to the motor and making the rotor spin. De-energizing the solenoid disconnects the motor.
A solenoid and contactor are used to avoid the need for an unusually large ignition switch and thick wires that would be required to handle the motor's high current demand directly. The solenoid requires a much lower current to energize.
Another crucial function of the solenoid is to slide and engage the pinion with the flywheel before the motor starts. A helical spline facilitates a slight rotation of the pinion as it slides, ensuring a more convenient meshing with the flywheel. Once meshed, the contactor closes, and the motor spins.
Due to the small size of the pinion relative to the flywheel, a gear ratio of 15:1 to 20:1 is typical. To generate sufficient torque to crank the engine, a planetary gear set is introduced. Planetary gears are efficient for large gear reductions within limited space, consisting of a sun gear, ring gear, carrier, and planet gears. In starters, the ring gear is fixed, the sun gear is the input, and the planet carrier is the output, providing a significant gear ratio, such as 7:1.
Once the engine starts, the pinion must disengage from the flywheel to prevent motor damage from excessive back-drive speed. This happens when the ignition switch is released, de-energizing the solenoid. To prevent damage if the operator fails to release the switch promptly, a one-way clutch is incorporated. This clutch uses rollers between outer and inner races to transfer power to the pinion when the motor spins. When the engine starts and drives the pinion at high speed, the rollers release, stopping power transfer and protecting the motor.