Chapter 14: Aviation Gasoline (AVGAS) and Combustion

Technical General for Aviators — Capt. Pankaj Pahil

14.1 Aviation Fuel (AVGAS)

• Piston engine aircraft use a gasoline fuel known as AVGAS52.
•  Grades and Colors: AVGAS is graded by its octane rating and is color-coded for
• identification.
• AVGAS 100LL (Low Lead): Blue, 100/130 rating.
• AVGAS 100 (High Lead): Green, 100/130 rating.
•  Volatility: This is the fuel's ability to vaporize. High volatility is needed for starting, but too
• much can lead to
• vapor lock (fuel boiling in the lines) or carburetor icing53. As altitude increases, atmospheric
• pressure decreases, and the fuel's boiling point lowers54.
•  Octane Rating: This is a measure of the fuel's resistance to detonation (knocking)55. It is

• not a measure of the fuel's energy content56. A higher octane rating allows for higher
• compression ratios and more power without detonation57. The rating is based on a
• comparison with reference fuels: Iso-Octane (rated 100) and Normal Heptane (rated 0)58.

14.2 Abnormal Combustion

•  Detonation: This is a violent, uncontrolled explosion of the remaining unburnt fuel/air
• mixture ('end gas') in the cylinder after normal ignition59. It is caused by excessive heat and
• pressure. The flame front can accelerate to over 1,000 feet per second60.
• Effects: Detonation causes a sharp rise in cylinder pressure and temperature, leading to a
• metallic knocking sound ('pinking'), power loss, and can cause severe engine damage like
• burnt pistons and valves61616161.
• Causes: High charge temperature, incorrect (too advanced) ignition timing, poor cooling, and
• using a fuel with too low an octane rating626262626262626262.
•  Pre-Ignition: This is the ignition of the fuel/air mixture before the spark plug fires63. It is
• usually caused by a local hot spot in the combustion chamber, such as an incandescent carbon
• deposit or an overheated spark plug electrode64. It leads to rough running and significant
• power loss.
•  Fuel Additives: Tetra Ethyl-Lead (T.E.L.) is added to fuel to increase its octane rating and
• prevent detonation65.