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Starter Motor: Function, Design & How It Works (Engine Part Explained)

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Last Updated on: 7th September 2023, 12:36 am

The starter motor is an electric motor designed specifically for the purpose of initially cranking over an internal combustion engine.

Its primary function is to rotate the engine’s crankshaft, which in turn sets the entire engine mechanism in motion.

This article explains the inner workings of the starter motor, what they are and how they work.

starter motor

How Starter Motors Work

The starter motor consists of many components, including an electric motor, a solenoid, a pinion gear, and a drive assembly.

  • The electric motor converts electrical energy from the vehicle’s battery into rotational force.
  • The solenoid acts as a relay, engaging the pinion gear with the engine’s flywheel when the ignition key is turned. This engagement enables the pinion gear to mesh with the flywheel’s teeth, allowing the starter motor’s rotational force to be transmitted to the engine.
  • As the starter motor activates, the pinion gear’s engagement with the flywheel causes the crankshaft to turn. This rotation initiates the intake, compression, power, and exhaust strokes within the engine cylinders, effectively starting the combustion process.
  • Once the engine is running and the ignition key is released, a mechanism within the solenoid disengages the pinion gear from the flywheel, allowing the engine to operate independently without interference from the starter motor.


There are many parts of a starting motor, the major parts are described below.

  • Armature: The armature constitutes an electromagnet positioned on the drive shaft, supported by bearings. Constructed from laminated soft iron, this core is enveloped by numerous loops or windings of conductive material.
  • Commutator: Situated at the rear of the housing, the commutator serves as a segment of the shaft where brushes come into contact to facilitate electrical conduction. Comprising two plates fastened to the armature’s axle, these plates establish the two connections for the electromagnet’s coil.
  • Brushes: The brushes run on a specific portion of the commutator located at the housing’s rear, making contact with its surfaces and thereby enabling the flow of electricity.
  • Solenoid: The solenoid consists of a pair of wire coils wound around a movable core. Functioning as a switch, the solenoid closes the electrical circuit, connecting the starter motor to the vehicle’s battery.
  • Plunger: Using the vehicle’s battery and the solenoid, the plunger is propelled forward, effectively engaging with the pinion.
  • Lever Fork: Linked to the plunger, the lever fork moves forward in tandem with the plunger’s motion. This sequence of actions subsequently sets the pinion into motion.
  • Pinion: The pinion is a combination of gears and springs which is activated upon engagement of the starter. As the gear extends into the gearbox housing, it meshes with the flywheel, initiating the rotation of the engine and commencing the combustion process.
  • Field Coils: Within the housing, the starter fields are securely contained using screws. These can comprise a configuration of two to four field coils linked consecutively. When energized by the battery, these coils transform into electromagnets, subsequently setting the armature into motion. This motion induces a magnetic field around the armature coils.
starter motor parts diagram

Evolution & Technology

From the early days of hand-crank starters, which required physical effort to crank the engine over for initial starting, to the introduction of electric starters.

Modern innovations include reduction gear starters, gear-integrated starter generators, and start-stop systems.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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