How a Car Starter Works
- The average starter uses seven major parts: a front and rear case that holds everything inside, the overrunning clutch or Bendix drive gear assembly, the rotating armature in the main electric motor, the field coil that goes around the armature and the solenoid mechanism. The starter as a whole is basically just a very powerful and very specialized electric motor that uses a few extra parts to enhance longevity and power. Some starters also use a gear drive just before the gear mechanism, which allows for more starting torque with a smaller motor.
- The starter solenoid is the smaller cylinder that sits on top of your main starter motor, and it's a surprisingly clever little mechanism. The solenoid is a sort of linear actuator, similar in principle to an electric motor, except that it goes back and forth instead of round and round. The solenoid has two basic parts: the field coil that goes around the center pipe-like plunger, and the plunger itself. When you turn the ignition key, voltage flows to the coil and turns it into an electromagnet, which pulls back on the plunger shaft. The plunger shaft movement itself is important -- as you'll see in a moment -- but the plunger movement also does something else.
- Older starters used solenoids -- or very powerful relays -- to send electricity to the starter. These remote-mounted relays used a field coil to pull on a spring-metal arm, which closed a circuit to send power to the starter. Modern starters, however, use the aforementioned solenoid mechanism itself as the relay. When the solenoid plunger pulls back, a copper washer on the back closes the contacts between the power supply from the battery and the power supply to the motor. The effect is the same as using a separate relay, but the whole thing is contained in the starter itself.
- Once power flows through those contacts, it flows into the commutator brushes on the armature. From here, the starter works just like any other direct-current electric motor. The commutator -- which looks like a ring cut into sections -- delivers power to the coils inside the armature via the brushes connected to the positive and negative power supplies. Each section on the commutator connects to a different coil of wire on the armature, so, as the shaft spins, the polarities of the armature magnets switch. These switching polarities push on a fixed magnetic field provided by the field coils inside the starter housing. The field coils are themselves electromagnets, since permanent magnets powerful enough for this duty would be far too heavy and expensive to put in every car on Earth.
- At this point, the starter motor is spinning, but the gear on its tip hasn't engaged the flywheel. The gear and over-running clutch assembly ride on grooves in the armature shaft, allowing them to slide forward and backward. When the solenoid's plunger pulls into its housing, it actuates a forked lever arm that pushes the starter gear forward toward the flywheel. The starter's over-running or sprag clutch mechanism allows the starter to drive the engine, but not the engine to drive the starter. If you've ever tried to start a car that's already running, then you may have heard a grinding, followed by a distinct whir. The grinding noise was the sound of the starter gear engaging the flywheel, and the whirring is the sound of the starter's sprag clutch.