Piston Engine Fuel Properties and Management

Aviation Fuel Properties

• 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.