FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationHard
A mechanic is investigating engine damage after a pilot inadvertently used a lower octane fuel than specified for a high-compression reciprocating engine. Which condition most likely caused the damage?
- AVapor lock in the fuel lines during climb
- BDetonation caused by uncontrolled, explosive combustion ahead of the normal flame front
- CHydraulic lock from liquid fuel entering a cylinder
- DExcessive carburetor icing due to fuel evaporation
Show answer & explanationAnswer & explanation
Correct answer: B. Detonation caused by uncontrolled, explosive combustion ahead of the normal flame front
Fuel with an octane rating lower than specified is more prone to autoignition under high cylinder pressures and temperatures, resulting in detonation—an uncontrolled explosion of the unburned charge ahead of the advancing flame front. This produces damaging pressure spikes that can crack pistons, damage rings, and burn valves.
Why the other options are wrong
- A. Vapor lock relates to fuel boiling in the lines, unrelated to octane rating.
- C. Hydraulic lock occurs from liquid fuel/oil in a cylinder during shutdown, unrelated to octane.
- D. Carburetor icing results from evaporative cooling, not fuel octane rating.
Detonation from Low-Octane Fuel
Uncontrolled, explosive combustion of the fuel-air charge ahead of the normal flame front, caused by using fuel with insufficient antiknock rating for the engine's compression ratio, leading to severe pressure spikes and potential engine damage.
- Low octane fuel increases susceptibility to autoignition
- Produces audible knocking and rapid cylinder pressure rise
- Can cause piston, ring, and valve damage if uncorrected
Memory trick: Low octane = short fuse for an engine explosion.