Hi all
Fuels are always a moving target, mostly due to regulations and changes in the fuel supply. Depending on where you are simply having the correct octane may not be good enough. Auto fuels in the past were very good to use and we normally could depend on the engines to not display any problems. Currently that is not the case, at least in the USA. The Ethanol lobby and other groups claiming clean air is worth every penny you pay have changed all this. We have also supply issues due to global conflicts and political patronage of the corn lobby. Now that I have vented a bit we should discuss what the engine needs.
Simply speaking the engine requires a fuel that will prevent knock due to pressure and heat on the fuel. Low temperatures play a role in fuel design and you must account for starting in cold temperatures. In many countries where this is an issue, like the USA and Canada for example, fuel is formulated based on the predicted weather patterns from month to month. In general around the middle of September in the northern hemisphere we blend in more Butane to the fuel to make is more volatile in these cold conditions and aid in starting. This works well until the temperature rises and then the fuel tends to boil off excessively (vapor lock issues) so winter blends should never be used in hot conditions. Auto fuels are also not formulated for high altitudes. I would recommend flying about 8 to 10000 ft or higher you should use only Avgas. For years we got away with auto fuels because fuel suppliers keep the supplies changing based on the weather of the region. September the fuels generally went to winter blends and in April they would switch back to summer blends. The actual changes are seen in something called the Reid Vapor pressure of the fuel. This is just about as important as the octane however it is very difficult to check with out some special equipment. Normally race car people who are building engines will do this to verify the fuels are suitable for them to run when on a dyno.
https://www.youtube.com/watch?v=OA2X9FXrHF4
In the past few years the regulatory people have been pressured, at least in the USA, into allowing winter blend fuels to be used int he summer months to absorb the fuel supply...what does that mean. It means they are selling crap auto fuel with the wrong Reid Vapor pressure to you in the summer and it can vapor lock. This condition is much worse if you have carb engine because your fuel ios not pumped to the engine under enough pressure to resist evaporation, Let's remember that the carb engine has no actual fuel sensors. You have a fuel gauge that reads your volume and you might have a pressure gauge however at the low levels we use with a carb it is not enough to prevent vapor locking in hot or high altitude conditions in the summer. Basically never use winter blend auto fuels in the summer or risk vapor lock. The injected engine is better only in that is has a 3 bar, 45 lb pressure that helps avoid this however it is still a risk.
Avgas is constant. it will be harder to start in the winter but has very good resistance to vapor lock as the Reid Vapor pressure is the same all year round. It has 20% Toluene that prevents gums forming in the fuel system and lacks aromatics that tend to leave hard deposits in the engine. If you are using a leaded fuel, say 100 LL, the deposits are much less in the injected engines because they run so much leaner.
Rotax is working on evaluation of fuels however the fuel supply is changing very fast so it is not possible to check them all in a timely manner. Swift fuels have been check and are allowed. They are not FAA approved but do have ASTM standards they meet. I am sure that at some point G100UL will also be tested.
The best advice is to check with your regional fuel supplier and ask about octane, Reid vapor pressures and if you have seasonal fuels like in the USA. At least then you can make an informed pick for what is best for your engine.
Good luck on your quest
