FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A technician is reviewing the valve timing diagram for a reciprocating engine and notes that the exhaust valve opens well before bottom dead center (BDC) on the power stroke. What is the primary purpose of this early exhaust valve opening?
- ATo reduce cylinder head temperature during the intake stroke
- BTo allow exhaust gas pressure to begin scavenging the cylinder before the piston reaches BDC
- CTo improve fuel atomization in the intake manifold
- DTo increase compression ratio during the compression stroke
Show answer & explanationAnswer & explanation
Correct answer: B. To allow exhaust gas pressure to begin scavenging the cylinder before the piston reaches BDC
The exhaust valve opens before BDC (while cylinder pressure is still relatively high) to take advantage of that residual pressure to begin blowing down and scavenging exhaust gases from the cylinder, reducing the pumping work required during the exhaust stroke. This early opening slightly reduces the effective power stroke but improves overall scavenging efficiency.
Why the other options are wrong
- A. Cylinder cooling during intake is affected by valve overlap, not early exhaust opening specifically.
- C. Fuel atomization occurs in the induction system, unrelated to exhaust valve events.
- D. Compression ratio is determined by piston/cylinder geometry, not exhaust valve timing.
Early Exhaust Valve Opening
The exhaust valve opens before BDC on the power stroke to use residual cylinder pressure for initial scavenging, reducing pumping losses during the exhaust stroke.
- Occurs before BDC while cylinder pressure is still elevated
- Reduces exhaust stroke pumping work
- Sacrifices small amount of power stroke for better scavenging
Memory trick: Exhaust valve opens EARLY to let the pressure blow itself out before the piston even asks.