FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A mechanic is checking the magneto-to-engine timing (external timing) on a reciprocating engine. At what point relative to the piston's travel should the magneto's contact breaker points open to produce a spark for a specific cylinder?
- AA specified number of degrees Before Top Dead Center (BTDC) on the compression stroke.
- BA specified number of degrees After Top Dead Center (ATDC) on the power stroke.
- CExactly at Bottom Dead Center (BDC) on the intake stroke.
- DExactly at Top Dead Center (TDC) on the compression stroke.
Show answer & explanationAnswer & explanation
Correct answer: A. A specified number of degrees Before Top Dead Center (BTDC) on the compression stroke.
Ignition must occur before the piston reaches Top Dead Center (BTDC) on the compression stroke. This allows sufficient time for the fuel/air mixture to ignite and achieve maximum pressure just after TDC, providing the most efficient power stroke. The exact timing (degrees BTDC) is specified by the engine manufacturer.
Why the other options are wrong
- B. Sparking ATDC on the power stroke would be far too late, resulting in significant power loss and potential engine damage.
- C. Sparking at BDC on the intake stroke is completely incorrect and would not result in combustion.
- D. Sparking exactly at TDC would result in inefficient combustion because it takes time for the flame front to propagate.
Magneto-to-Engine Timing (External Timing)
The precise synchronization of the magneto's spark event with the piston's position and crankshaft rotation for optimal engine operation.
- Spark occurs BTDC on the compression stroke.
- Specific degrees BTDC are engine-manufacturer specified.
- Ensures maximum power and efficiency.
Memory trick: Spark before the peak, for power you seek.