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 engines are capable, or modified to run either fuel; although some with restrictions.

Purpose built aircraft engines are designed to use aviation gasoline and some types can run on auto fuel too. In this section we delve deeper into piston aircraft fuels, AVgas.

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 tetra-ethyl or alkyl-lead, anti-knock additives, metal de-activators, 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 gasoline's (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 anti-knock properties. Fuels with the same anti-knock properties as iso-octane are given a rating of 100. Another hydrocarbon with very poor anti-knock 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 anti-knock 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.

In 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 higher 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 of motor gasolines explains it all.

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 anti-knock agents, which delay the onset of detonation. Until recently, the most important such additive, for both automotive and aviation use, was tetra-ethyl lead (TEL). It's found in aviation fuels in the following proportions:

AVgas density

AVgas weighs around 6 Lbs/US gallon (to be more precise: 5.97 Lbs/US gallon or in other words: 0.719 g/mL).

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 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.

For the read hungry: ConocoPhilips published a document on AVgas fuel specifications, it opens in a new window