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 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).

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 knocking inside the engine during combustion. Thus higher compression (more power) engines could be used.

Use of alternate fuel grades
Before using an alternate grade for AVgas 80/87 or even using automotive fuel gasoline in your aircraft read AC91-33A from the FAA. Opens in a new window.

AVweb wrote an article on the use of Mogas in aircraft engines. The EAA has dedicated a website on aviation fuels.
The Dutch Aviation Authorities published a document about the use of Mogas containing bio-alcohol, interesting read for those running with Rotax engines.

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

Mogas Types
Chevron has created a rather technical document about motor gasolines. This will open in a new window.

Testing for Alcohol in Mogas

The following steps describe how to test if there is any alcohol in your Mogas:

• Using a glass or chemical resistant plastic (such as TPX) container, mark ten equally spaced volumes. A graduated cylinder is ideal; however, a non-tapered glass jar, such as a large (quart) olive bottle, will work.
• Add one part of water (approximately 100 ml) into the container, fill to the first mark, and then add nine parts (approximately 900 ml) of automotive gasoline, fill to the top mark. Shake thoroughly, let stand for 10 minutes or until automotive gasoline is again bright and clear. Record the apparent level of the line between the automotive gasoline and water.

Interpreting the above test:

• If alcohol is present in the automotive gasoline, the water will absorb it, and the amount of water will appear to increase, indicating the automotive gasoline should not be used in the aircraft.
• However, if the water level remains the same, no alcohol is present in the automotive gasoline and it can be used in the aircraft.

Operators of aircraft approved for operation with Mogas containing methanol or ethanol shall consider the lower energy content of such fuel (higher fuel consumption).