FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A pilot reports that during climb power on a hot day, an engine began running extremely rough with a rapid rise in cylinder head temperature, occurring even before the fuel/air mixture would normally be ignited by the spark plugs. A carbon deposit acting as a glowing hot spot in the combustion chamber is suspected. This condition is best described as:
- APre-ignition, where the mixture ignites from a hot spot before the timed spark occurs
- BBackfiring, caused by a lean mixture igniting in the intake manifold
- CAfterfiring, caused by unburned fuel igniting in the exhaust system
- DDetonation, an uncontrolled explosive burning of the mixture after normal spark ignition
Show answer & explanationAnswer & explanation
Correct answer: A. Pre-ignition, where the mixture ignites from a hot spot before the timed spark occurs
Pre-ignition occurs when the fuel/air mixture ignites from a source other than the timed spark, such as a glowing carbon deposit or overheated spark plug electrode, before the spark plug fires. This is distinct from detonation, which is an uncontrolled explosive combustion of the end-gas that occurs after normal spark ignition has already started the burn.
Why the other options are wrong
- B. Backfiring involves ignition in the intake manifold, not a hot-spot ignition in the cylinder before spark.
- C. Afterfiring occurs in the exhaust system, unrelated to this in-cylinder pre-spark ignition scenario.
- D. Detonation happens after normal ignition, not before it as described here.
Pre-ignition
Ignition of the fuel/air charge by a hot spot (such as a carbon deposit or overheated plug) before the timed spark occurs, distinct from detonation which follows normal ignition.
- Caused by hot spots: carbon deposits, overheated plugs, sharp edges
- Occurs before the timed spark fires
- Can be triggered or worsened by prior detonation damage
Memory trick: Pre-ignition jumps the gun before the spark even fires!