A starter motor is the electric machine that gets an internal combustion engine moving from rest. When the driver turns the key or presses the start button, the battery sends a large current to the starter for a short time. The starter spins a small pinion gear that meshes with the engine flywheel, cranking the crankshaft fast enough for air, fuel, and spark to begin combustion.
Once the engine runs on its own, the starter must disengage quickly to avoid damage.
Key Facts
- Electrical power from the battery is P = VI, where V is voltage and I is current.
- A typical 12 V starter may draw 100 A to 300 A or more during cranking.
- The solenoid acts as both a heavy-duty switch and a mechanical actuator for the pinion gear.
- Torque is twisting force, and for a rotating part τ = Fr.
- The pinion gear is small and drives the larger flywheel, increasing torque at the engine crankshaft.
- The starter operates only briefly because high current causes heating, with heat related to P_loss = I^2R.
Vocabulary
- Starter motor
- An electric motor that briefly turns the engine crankshaft so the engine can begin running by combustion.
- Solenoid
- An electromagnetic switch that moves a plunger to engage the starter gear and connect high current to the motor.
- Pinion gear
- The small gear on the starter motor that meshes with the flywheel teeth during cranking.
- Flywheel
- A heavy rotating wheel attached to the engine crankshaft that helps smooth engine motion and provides teeth for the starter to engage.
- Torque
- A measure of rotational turning effect that depends on force and the distance from the rotation axis.
Common Mistakes to Avoid
- Confusing voltage with current, because the 12 V battery voltage is not what makes the starter powerful by itself. The starter needs very large current to deliver high electrical power.
- Thinking the starter keeps the engine running, because it only cranks the engine before combustion becomes self-sustaining. After startup, fuel combustion supplies the engine power.
- Ignoring the solenoid, because the starter motor does not simply spin whenever the key is turned. The solenoid first engages the pinion gear and then closes the high-current circuit.
- Holding the starter on too long, because the motor is designed for short bursts. Long cranking can overheat windings, drain the battery, and damage the pinion or flywheel teeth.
Practice Questions
- 1 A 12 V battery supplies 180 A to a starter motor while cranking. What electrical power is delivered to the starter in watts?
- 2 A starter pinion applies a 220 N tangential force at a radius of 0.030 m. What torque does the pinion produce?
- 3 Explain why the starter pinion must disengage after the engine starts, and describe what could happen if it stayed meshed with the flywheel.