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Question

Difficulty: MediumStarting and Charging Systems

During engine cranking, the starter pinion gear engages the flywheel ring gear to turn the crankshaft. Once the engine starts and begins running under its own power, which component protects the starter motor by preventing the engine flywheel from spinning the starter armature at destructive speeds?

  1. A
    Rectifier bridge assembly
  2. Overrunning clutchAnswer
  3. C
    Solenoid pull-in winding
  4. D
    Alternator stator winding

Answer

The overrunning clutch prevents the running engine from spinning the starter motor armature at excessive speeds.
The overrunning clutch (also known as a one-way clutch or starter drive clutch) allows torque to flow in only one direction: from the starter armature to the flywheel ring gear. The moment the internal combustion engine fires and begins spinning faster than the starter motor, the internal rollers or sprags release, allowing the pinion to spin freely (freewheel) until the solenoid disengages the pinion from the flywheel.

Step-by-Step Solution

1
Identify the mechanical challenge present when an internal combustion engine starts.
The gear ratio between the starter pinion and the flywheel ring gear is roughly 15:1 to 20:1. When the engine starts and revs to 1,000 RPM, the starter motor would be forced to spin at 15,000 to 20,000 RPM if mechanically locked.
Such high rotational speed would cause centrifugal force to throw the starter armature windings and destroy the motor instantly.
2
Determine which starting system component provides one-way torque transfer.
The overrunning (one-way) clutch in the starter drive locks in one direction to crank the flywheel, but freewheels when the engine flywheel turns faster than the starter motor.
This mechanical decoupling disengages torque transfer from the engine back to the starter armature until the pinion fully retracts.

Key Concept

Starter motor drive protection and one-way overrunning clutch operation
Estimated Time:1m 0s
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Topics

Starting and Charging Systems

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