Aviation gasoline is a high octane fuel used to run precision engines like those found in aircraft, elite racing cars and some boats. With an octane rating of 100, versus a rating between 95 and 98 for everyday automobile vehicles, it is commonly called avgas. This fuel also has a lower volatility, meaning that it can withstand higher temperatures before igniting and can run smoother than automotive gasoline, which is important for high altitude use. In aviation circles, avgas is used to describe aviation gasoline and mogas is used for motor or automobile gasoline.
Developed in the 1940s, aviation gasoline was used for commercial and military purposes. During World War II, it was used in Spitfire and Hurricane fighters, Mosquitos, and Lancaster bombers. Interestingly, the mixtures used today are very similar to those developed for the war effort.
Avgas is now available in various grades dependent on the lead concentration of the fuel. The addition of lead, in the form of Tetra-ethyl-lead (TEL), has been used to promote higher octane ratings. Now known to be toxic, TEL has been phased out for automobile use and is used sparingly to improve the octane readings for aircraft.
The different grades of avgas are usually identified by two numbers. These two numbers make up the Motor Octane Number (MON) for the fuel. The first number denotes the octane rating of the fuel when it was tested using the aviation lean setting, or when not enough fuel is added to the fuel-air mixture. The second number gives the octane reading for the aviation rich setting, or when too much fuel is found in the fuel-air mix. These settings attempt to simulate conditions that may occur during flight.
Unique dyes are used to identify the different grades of aviation gasoline. Popular in the late 20th century, 80/87 was dyed red. Due to its high lead content, it has been phased out and is available on a very limited basis. Green dye is used for aviation gasoline 100/130, which has a much higher octane reading. The most commonly used aviation gasoline is known as one hundred low lead (100LL), which is dyed blue and conforms to strict guidelines for lead additives.
The largest users of aviation gasoline are North America, Australia, Brazil and Africa. The consistent rise in aviation gasoline prices has spurred an investigation into converting the aviation industry to using common diesel fuel. Its relatively inexpensive distillation process, lower cost, and similarity to avgas makes it a desirable alternative.