By harnessing the power of the battery, the starter motor powers the car to "turn over," or start.
The starter motor is the heart of the starting system. It is a very powerful DC electric motor with a gear system that connects to the flywheel on the engine. The starter motor causes the engine to revolve at about 350–450 rpm and the engine develops enough vacuum to enable combustion to occur, which is what causes the engine to start. Once the engine starts, the driver releases the ignition key from the start position, allowing the starter to disengage.
The two very large battery cables are connected directly to the starter motor. The positive cable connects to the motor feed while the negative cable connects near the starter motor case.
The ignition switch provides a trigger signal when the key is turned to the start position. This trigger signal is sent to a heavy duty switching device on the starter motor called the starter solenoid. The trigger signal from the ignition switch energizes a magnetic field that brings together two large copper discs that allow the battery to feed current into the armature of the starter motor, causing it to rotate or 'crank' the engine. There is a specialized gear engagement mechanism on the end of the starter motor's armature shaft called the starter drive. When the starter motor spins, it extends a gear that engages the flywheel so the starter motor is able to rotate the crankshaft.
When the engine 'fires' it spins the starter motor gear, this gear has a special releasing mechanism called an over-running clutch. The over-running clutch allows the starter gear to spin freely on its shaft while the engine picks up speed as it begins to run. Otherwise the engine could destroy the starter motor and drive by turning it too violently and too fast. When the driver of the vehicle releases the ignition key, the trigger signal is interrupted to the magnetic field in the starter solenoid and halts the operation of the starter motor.