Common Aviation Fuels, AVgas
Experimental aircraft commonly use engines which consume AVgas (Lycoming / Continental / Franklin and equivalent types) or engines running Mogas (Rotax, Subaru etc).
Some of these aero engines are capable, or modified to run either fuel; although with restrictions or with a special type certificate.
Purpose built aircraft engines are designed to use aviation gasoline and some types can run on auto fuel too, usually after replacing alcohol sensitive fuel system parts.
AVgas has long been used as the fuel for piston powered aircraft, but as we will see, aero diesel engines will use either JET or diesel fuels.
This development is in anticipation of the disappearance of high priced AVgas in the near future, or until a good replacement fuel is found and worldwide delivery and quality is assured.
In this section we delve deeper into the most common spark ignited aircraft fuel, AVgas, taking a look into its properties and advantages.
Aviation Gasoline
Aircraft piston engines operate using the same basic principles as spark ignition engines of cars, but they have a much higher performance requirement. Aircraft engines are designed to run at 55% power or more (on take off even 100%), where as car engines run at an average of 30% power or less. The design of the aero engine is different in terms of strength: think of cylinders, pistons, bearings, crankshaft etc, etc.
AVgas is gasoline fuel developed for reciprocating piston engined aircraft. Common additives to AVgas include tetraethyl or alkyl-lead, antiknock additives, metal deactivator, color dyes, oxidation inhibitors, corrosion inhibitors, icing inhibitors, and static dissipaters. It is very volatile and extremely flammable at normal operating temperatures. Proper and safe handling of this product is therefore of the highest importance. AVgas grades are defined by their octane rating. Two ratings are applied to aviation gasolines (the lean and the rich mixture rating) resulting in a multiple numbers e.g. AVgas 100/130 (lean mixture is 100 and the rich mixture is 130).
Octane and lead
Gasolines are formulated from hydrocarbons, one of them is iso-octane with excellent antiknock properties. Fuels with the same antiknock properties as iso-octane are given a rating of 100. Another hydrocarbon with very poor antiknock properties is heptane which mixed with iso-octane in varying amounts to give the reference fuel an octane rating with which fuels are compared to measure its antiknock quality.
The addition of lead (or other replacements) gives fuel the possibility to produce more power before detonation occurs, for example in a higher compression engine. If power produced by pure fuel is 100 then the addition of lead will let the power increase up to 45%, thus the performance number is 145. The fuel air ratio (lean or rich mixture) also has an important influence on the power produced.
Contrary to popular belief, just changing to a fuel with a higher octane without changing anything else will not make an engine produce more power. The higher octane value is important in high compression engines where the octane delays the possibility of detonation or knocking in the engine at high power settings where a lower octane fuel would not. Page 6 in Chevron's Motor Gasolines explains it all in detail.
AVgas types
AVgas 100, high lead - colored green
The standard high lead (1 gr/liter) high octane fuel for aviation piston engines. There are two specifications for AVgas 100. The ASTM D 910 and UK DEF STAN 91-90. These are almost the same but have some differences in antioxidant content, oxidation stability requirements and lead content.
AVgas 100LL, low lead - colored blue
Low lead version of AVgas 100. Still containing about 0.5 gr lead per litre of fuel, low lead is a relative term. This grade is listed in the same specifications as AVgas 100, ASTM D 910 and UK DEF STAN 91-90.
AVgas 82 UL, unleaded - colored purple
A relatively new grade targeted at the low compression ratio engines not needing high octane AVgas 100(LL) and designed to run on unleaded fuel (0,1 gr/liter).
The octane rating can be increased beyond the simple proportion of octane to heptane by adding antiknock agents, which delay the onset of detonation. Until recently, the most important such additive, for both automotive and aviation use, was tetraethyl lead (TEL). It's found in aviation fuels in the following proportions:
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AVgas densityAVgas weighs around 6 Lbs/US gallon (to be more precise: 5.97 Lbs/US gallon or in other words: 0.719 g/mL) at standard temperature (15°C). 
More infoConocoPhillips published a document on AVgas fuel specifications, AeroShell also has a document about aviation fuels and this technical review from: Chevron Aviation Fuels |
Some history of AVgas
In the past, there were many different grades of aviation gasoline in general use e.g. 80/87, 91/96, 100/130, 108/135 and 115/145. Specifically designed for high powered turbo and supercharged radial engines. However, with decreasing demand these all have been narrowed down to one type, AVgas 100/130. Also known as AVgas 100.
Eons ago, an additional grade was introduced to allow one fuel to be used in engines originally designed for grades with lower lead contents: this was called AVgas 100LL, the LL standing for 'low lead'. Much later AVgas 82 UL was added.
Lead was added to increase the fuels resistance against detonation inside the engine during combustion. Thus higher compression (more power) engines could be used.
