LeSving wrote:
Mix of mogas and G100UL no problem then it seems.
See my post above. The STC is explicit that this is not permissible. Whether that’s just a formality or based on actual chemical incompatibility, who knows.
According to this wikipedia article, it is simply aviation alkylate with the TEL removed, then added xylene and mesitylene. Surely that can be blended with mogas, but obviously that blend cannot longer be used in an engine requiring 100 octane.
LeSving wrote:
According to this wikipedia article, it is simply aviation alkylate with the TEL removed, then added xylene and mesitylene. Surely that can be blended with mogas, but obviously that blend cannot longer be used in an engine requiring 100 octane.
G100UL is noticeably denser and has higher energy content per litre than regular 100LL, so there must be a substantial amount of xylene/mesitylene – not just used as additives.
It is possible to mix two types of fuel using different technologies to achieve a certain octane rating, and achieve an octane rating higher than either. This is the result of e.g. lead having a non-linear effect on octane, the first little bit added to unleaded fuel increases octane a lot. I think that’s likely why mixing 100UL and 100LL is OK but mixing 100UL and lower octane unleaded auto fuel is not.
Adding 25% 100LL to 100UL may result in a pretty high octane mix, for anybody who needs it. Cheaper than race fuel for high boost Reno racers 
LeSving wrote:
it is simply aviation alkylate with the TEL removed, then added xylene and mesitylene. Surely that can be blended with mogas, but obviously that blend cannot longer be used in an engine requiring 100 octane.
Our (non-certified) Lycoming would be happy with 91/96 UL, so if/when 100UL will be on tap, it would be great if I could mix any combination of 91/96, 100LL, 100UL, and non-alcoholic Mogas … pipe dream?
I really can’t understand why Conti/Lyco are still selling engines where the carb floats, seals etc are none tolerant to ethanol.
Surely it can’t be that difficult?
Airborne_Again wrote:
The STC is explicit that this is not permissible.The FAQ in their powerpoint (available from the hjelmco website, see link some pages ago here) says it’s ok, so not clear at this point I guess.
Sebastian_H wrote:
if/when 100UL will be on tap, it would be great if I could mix any combination of 91/96, 100LL, 100UL91/96UL is an avgas, so it’s fine.
Arne wrote:
The FAQ in their powerpoint (available from the hjelmco website, see link some pages ago here) says it’s ok, so not clear at this point I guess.
It’s more likely that I misread the STC text which says “CAUTION: DO NOT MIX G100UL AVGAS WITH MOTOR GASOLINE (MOGAS)”
…but then continues on the next line “THAT CONTAINS ETHANOL OR METHANOL.”
My bad. Sorry for the confusion.
OTOH, how common is MOGAS without any addition of ethanol these days?
In Germany, most airfields that sell Mogas make extra sure that it is of the ehtanol-free type, because their customers are not only the Rotax crowd, but also Lycoming/Continental crowd with the approapriate STCs.
It is the old debate whether MOGAS is “car petrol”.
I think the intention is that it is a quality controlled version of car petrol, but that is not how many/most use the term.
Here, car petrol is 5% ethanol and going to 10% soon, and “mogas” means filling up jerrycans at a petrol station. Where does MOGAS actually exist?
