Octane Rating: Indicates the fuel's resistance to detonation.
Higher Octane: Higher resistance to detonation (burns slower/smoother).
Lower Octane: Lower resistance.
Usage: Usually safe to use a higher grade than recommended (e.g., using 100LL instead of 80/87), but strictly forbidden to use a lower grade.
Contamination: Water is the most common contaminant.
Can accumulate from condensation (empty tanks overnight) or leaks.
Prevention: Fill tanks overnight to minimize air space for condensation.
Check: Drain fuel sumps before the first flight of the day to remove water/sediment.
Abnormal Combustion
Detonation ("Engine Knock")
Definition: Spontaneous explosion of the fuel/air mixture inside the cylinder (instead of a smooth burn).
Effects:
Exerts excessive force/shock load on piston and cylinder head.
Increased noise ("knocking") and vibration.
High Cylinder Head Temperature (CHT).
Loss of power.
Engine damage (piston/ring failure) if severe.
Causes:
Too low octane fuel.
High Manifold Pressure (MAP) with Low RPM.
High Cylinder Head Temperature.
Too lean mixture.
Corrective Actions:
Reduce Manifold Pressure (Retard Throttle).
Enrich Mixture (Fuel cools the cylinder).
Increase Airspeed (Improve cooling).
Pre-Ignition
Definition: The mixture is ignited prematurely (before the spark plug fires) by a hot spot in the cylinder (e.g., carbon deposit, glowing spark plug tip).
Effects:
Rough running.
Power loss.
Further overheating.
Can cause the engine to continue running after ignition is switched off (Dieseling).
Difference from Detonation: Detonation is an explosion after ignition; Pre-ignition is an ignition before the spark.
Vapour Lock
Definition: Fuel vaporizes in the feed lines, creating bubbles that block the flow of liquid fuel.
Cause: High fuel temperatures, low pressure, or high altitude.
Prevention: Booster pumps (low-pressure pumps) in the tanks pressurize the fuel lines to suppress vaporization.