The Piston Engine

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.

 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.