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 operating 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 drop-in replacement fuel is found and worldwide delivery and quality is assured.
In this section we delve deeper into the most commonly used spark ignited aircraft fuel, AVgas, taking a look into its properties and advantages.
Aircraft piston engines operate using the same basic principles as the spark ignited engines used in cars, but with a much higher performance requirement. They are designed to run at 55% power or more (on takeoff even 100%) continuously, where as car engines run at an average of 30% power or less. The design of an aircraft engine is different in terms of strength: think of cylinders, pistons, bearings, crankshaft etc, etc.
AVgas is gasoline fuel developed for reciprocating piston engines. Common additives 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. The 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).
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 these days) gives the engine the ability to produce more power before detonation occurs, for example with higher compression types. If power produced by pure fuel is 100 % then the addition of lead might let the power increase up to 145 %, 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.
100, high lead - colored green
The standard high lead (1 gr/liter) high octane fuel for piston engines. There are two specifications: the ASTM D 910 and UK DEF STAN 91-90. These are almost alike but have some differences in antioxidant content, oxidation stability requirements and lead content.
100LL, low lead - colored blue
Low lead version. But still contains some 0.5 gr lead per liter 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.
82 UL, unleaded - colored purple
A relatively new grade targeted at the low compression ratio engines not needing high octane 100LL 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 is found in these fuels in the following proportions:
The relative weight is 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).
In the past, there were many different grades of AVgas 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, 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 100LL, the LL standing for 'low lead'. Much later 82 UL fuel was added to the family.
Lead increases the resistance against detonation inside the engine during combustion. Thus higher compression (more power) engines could be used.
Unleaded fuel is used today because of health concerns due to the millions and millions of cars using that fuel. The number of aviation engines using AVgas is a very small fraction compared to the amount of car engines in use, so there should be no problem. Even still, the aviation industry is slowly moving to unleaded fuel as engine life is improved by not using lead (among other benefits).