Avgas

Avgas is a high-octane fuel used for aircraft and, in the past, racing cars. Avgas includes a number of additives in order to decrease volatility so it won't evaporate away as quickly, which is important for high-altitude use. The particular mixtures in use today are the same as when they were first developed in the 1950s and 60s, and therefore the high-octane ratings are achieved by the addition of tetra-ethyl lead, a fairly toxic substance that was phased out for car use in most countries in the 1980s.

Avgas is currently available in several grades with differing lead concentrations. 80/87 has the lowest lead content at 0.5 grams lead per gallon, and is only used in very low compression ratio engines. Avgas 100/130 is a higher octane grade aviation gasoline, containing about 4 grams of lead per gallon. 100LL “low lead" was designed to replace Avgas 100/130. Avgas 100LL contains about 2 grams of lead per and is the most commonly available and used aviation gasoline.

Many general aviation aircraft engines were designed to run on 87 octane, the standard for automobiles today. Direct conversions to run on automotive fuel, known as mogas in the industry, are fairly common. However the alloys used in aviation engine construction are rather outdated, and engine wear in the valves is a serious problem on mogas conversions. Additionally, the wider range of allowable vapor pressures found in MOGAS pose some risk to aviation users if fuel system design considerations are not taken into account. MOGAS can vaporize in fuel lines causing a vapor lock (a bubble in the line) starving the engine of fuel. This does not constitute an insurmountable obstacle, but merely requires examination of the fuel system to ensure adequate shielding from high temperatures and the existence of sufficient pressure and flow in the fuel lines.

More information on AVGAS and MOGAS comparisons are available from the Experimental Aircraft Association (EAA), which has produced a number of Supplemental Type Certificates (STCs) for the use of MOGAS in aircraft. (See http://www.eaa.org/education/fuel )

In Europe avgas prices are so high that the entire General Aviation (GA) industry is being wiped out. There are a number of efforts to convert the industry to diesel instead, which is common, inexpensive and has a number of advantages for aviation use. It remains to be seen if there is enough of a market left for this to occur.