FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A technician studying a valve timing diagram notes that the intake valve closes considerably after bottom dead center (BDC) on the compression stroke rather than exactly at BDC. What is the primary reason for this delayed intake valve closing?
- ATo take advantage of the momentum of the incoming fuel/air charge, improving cylinder filling at operating RPM
- BTo reduce exhaust valve overlap and prevent backfiring
- CTo allow additional time for spark plug firing before compression begins
- DTo reduce the compression ratio for smoother idling
Show answer & explanationAnswer & explanation
Correct answer: A. To take advantage of the momentum of the incoming fuel/air charge, improving cylinder filling at operating RPM
At normal operating RPM, the incoming air/fuel mixture has inertia and continues flowing into the cylinder even after the piston passes BDC and starts upward. Closing the intake valve slightly late allows this momentum to continue filling the cylinder, improving volumetric efficiency before the valve finally seals for compression.
Why the other options are wrong
- B. This describes intake valve closing, not overlap, and is unrelated to backfire prevention.
- C. Spark timing is unrelated to intake valve closing timing.
- D. Compression ratio is a fixed geometric relationship set by cylinder/piston design, not altered by valve timing.
Delayed Intake Valve Closing (Valve Lag)
The intake valve remains open somewhat past BDC on the compression stroke to use the momentum of the incoming charge for improved cylinder filling (volumetric efficiency).
- Exploits inertia/momentum of intake charge (ram effect)
- Improves volumetric efficiency at normal operating RPM
- Part of the overall valve timing diagram along with overlap and lead
Memory trick: 'Momentum keeps filling even after BDC — don't shut the door too soon'