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Difficulty: MediumUpshifting and Double Clutching

A commercial driver is accelerating a tractor-trailer on a level highway and executing an upshift from fourth gear to fifth gear using a non-synchronized manual transmission. While the gearshift lever is in neutral and the clutch pedal has been momentarily released, what must happen before pressing the clutch again to complete the shift?

  1. A
    Depress the accelerator firmly to increase engine RPM to match transmission output speed
  2. Allow engine RPM and input shaft speed to drop to match the gear ratio required for the higher gearAnswer
  3. C
    Push the clutch pedal fully to the floorboard to engage the clutch brake while the vehicle is rolling
  4. D
    Hold the engine at governor maximum speed until road speed increases significantly

Answer

Allow engine RPM and input shaft speed to drop to match the gear ratio required for the higher gear
When shifting up to a higher gear, the engine must operate at a lower RPM to match the vehicle's road speed in that gear. Releasing the clutch in neutral couples the transmission input shaft to the decelerating engine, allowing both to slow down to the exact rotational speed needed for the next gear.

Step-by-Step Solution

1
Identify the mechanics of upshifting in an unsynchronized manual transmission
Higher gears require lower engine RPM for any given road speed compared to lower gears.
The gear ratio decreases in higher gears, meaning the engine turns fewer times per wheel revolution.
2
Analyze the neutral phase of the double-clutch upshift
Releasing the clutch pedal while in neutral connects the engine to the transmission input shaft without driving the wheels.
With the accelerator released, the engine and input shaft decelerate together toward the target RPM for fifth gear.
3
Determine the synchronization condition needed before completing the shift
Once engine RPM drops to match the new gear speed, the clutch is depressed again and the gear lever moves smoothly into fifth gear.
Matching rotational speeds prevents gear clash and enables smooth engagement.

Key Concept

Double Clutching RPM Synchronization during Upshifting
Estimated Time:1m 0s
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Topics

Upshifting and Double Clutching

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