Common Aviation Fuels, Mogas
Aircraft commonly have engines which consume AVgas (Lycoming / Continental / Franklin and equivalent types) or engines running on Mogas (Rotax, Subaru etc). Some are capable, or modified to run either fuel; although possibly with operating restrictions or maintenance limitations.
Today, Mogas can contain a certain percentage (3 - 5%, going up to 10% or more) of methanol or ethanol (bio-alcohol) and the possible effect of using that fuel in aviation engines must be considered by the pilot and that is the subject here.
Another issue is that the composition of Mogas (even the amount of alcohol) varies each and every day, whereas AVgas always has the same specified certified content.
Mogas in Engines
Generally, Mogas is cheaper than AVgas (Europe) which is one of the reasons pilots wanting to use that fuel. Even some manufacturers advise users to mainly use Mogas in their engines but that AVgas can be used as well. The Rotax four stroke models (912, 914) will run happily on AVgas, but due to the higher lead content oil changes must be done every 50 hours and must be mineral or semi synthetic in blend.
Nowadays, Lycoming is developing engines capable of running both fuels. Mainly because of the anticipated reduced availability of AVgas.
Running an engine on Mogas can introduce unwanted and unexpected side effects because of the possible blending with bio-alcohol. Gasoline pumps should be labeled as such if the fuel the pump dispenses contains bio-alcohol so that the buyer of the fuel is warned.
Bio-alcohol attracts, carries and retains water, this has number of side effects in the engine: bio-alcohol is absorbed by the water and is difficult to detect (see below). After engine shutdown water can lead to corrosion on vital parts. Water will freeze in cold conditions and during carburetion. Water also causes vapor lock and lowers the vaporization point of fuel. If the aircraft is then flown at higher altitudes (also favouring vapor lock) this could lead to an engine failure at the most unexpected time.
Other issues with bio-alcohol blended fuels are: lack of lubricity; alcohol is a solvent and could clean deposits in the fuel system and carry them to the filters clogging them. Bio-alcohol also burns leaner and may cause an increase in exhaust gas temperatures and possibly exhaust valve problems.
Composite fuel tanks
Some aircraft have tanks made from composites, and the alcohol component in Mogas slowly attacks these composites thereby dissolving the tank structure resulting in leaks. The solution is to either use a coating to protect the inside of the tank or use metal fuel tanks altogether. Make sure that all seals are resistant against alcohol. Vinylester resins have a much better resistance against these chemical products.
A number of manufacturers are advising against Mogas and recommend AVgas. For Rotax engines it means that using AVgas results in more maintenance due to the lead content and higher fuel cost for the owner.
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 (IVW/DL) published a document about the use of Mogas containing bio-alcohol, very interesting for those running Mogas with Rotax or any other aviation engines which may be run on that fuel.
Mogas types and properties
Chevron has published an in-depth technical document about Motor Gasolines engines and Mogas properties. A must read if you want to know it all about this fuel.
Operators of aircraft approved for operation with Mogas containing methanol or ethanol shall consider the lower energy content of such a fuel (which could result in a lower performance or higher fuel consumption).
Adding bio-alcohol (or ethanol) oxygenates the fuel (adds oxygen to the fuel) resulting in a more complete fuel burn. But the drawbacks are possible damage to rubber gaskets and composite fuel tanks as alcohol is a solvent.
Gasoline is a mix or blend of heptane and iso-octane (2,2,4-trimethylpentane) and contains no oxygen. High octane AVgas contains more iso-octane preventing engine knock. For fuel to burn you will need to add the right amount of oxygen (mixture). And during combustion oxygen will convert into CO2, which is plant food.
Bio-alcohol or ethanol contains oxygen and less carbon, resulting in lower energy levels (and higher fuel consumption during use). And the already added oxygen causes ozone formation during combustion in the cylinders resulting in increasing ozone pollution worldwide.
As a result these so called 'green fuels' are not so green after all. Plus they seem to be made from human edible food sources increasing those prices too. This is a path we should keep far away from.