912ULS, Using RON 98 Query

Hi Des,

"For what it’s worth and just to add to the “mix”,  here’s a blurb from B.P. Australia. They say, RON increases over time in an unsealed container !!!!"

I am not a chemical or mechanical engineer but have a vague understanding that the loss (or in BP's case gain) of % RON is not the full story -

As fuel ages, in an open to atmosphere container,  it loses some of its more volatile fractions - as I understand it these have more to do with things like, ease of starting, rather than anti knock/Octane (RON standing for Research Octane Number ).

The advice in the BP document appears a tad contradictory but I think some nuggets can be obtained:

BP:

The storage life of petrol is one year when stored under shelter in a sealed container. Once a seal is broken the fuel has a storage life of six months at 20°C or three months at 30°C.

Hmm! backs up/extends my earlier advice on storage but then goes on to say "Once a seal is broken the fuel has a storage life of six months.." which doesn't really match their advice on "storage life of petrol in equipment fuel tanks is one month"

 "The storage life of petrol in equipment fuel tanks is one month. This can be extended by topping up with one third of fresh fuel, which restores the volatile components that have evaporated."

I think it would be reasonable to substitute "equipment fuel tanks"  with aircraft fuel tanks. The 1/3 fresh nicely quantifies  & echoes my earlier advice.

"At the end of five weeks the fuel is 5% heavier and the fuel air mix will contain more fuel. This means your engine will run richer possibly resulting in fouled spark plugs and higher fuel consumption."

Clearly, aged fuel is not going to do good things for your Rotax, even if technically RON has actually increased.

When preparing to store a boat for extended periods e.g. two months or more, it is best to completely remove all fuel from the tank.

As little as "two months" I see a lot of small aircraft that do not get flown for very many months - I hope the pilots are aware of this advice.

Hi Sean,

Yes, as I also said, fresh fuel is key. Difficult to achieve at all times for private owners, fuel conditioner, (Sta-bil or Penrite Ultimate for example) is an option if you don’t mind going against the Rotax “no fuel additives” advice. 

Far from me to suggest it but a cynical view could also see how the fuel companies would like us all to replace our fuel with new as often as possible ? !

As a retired person, I get to fly quite frequently,  so fresh fuel not such a concern. Since converting GA/RAA ,about 15 years ago, I changed my refueling habit, to before, rather than after, flight.

Interesting that in the S Highlands you can still get 95 RON - it must be cracking on for 10 years, since our local servos (outer SW Sydney) supplied it.

Regarding additives - I am not keen on the idea of straying from Rotax advice.

On a tangent:

Additives; (mechanic in a bottle) - I periodically  "drench"  my diesels, with a fuel conditioner/ algal suppressor - I think/hope this achieves a prophylactic against the development of algae but only in diesel fuel systems.  Experience / decrepitude, has taught me that, for the most part fuel/oil additives are not cost effective (even if some do what they claim ?). I think I am correct when I say, several studies have demonstrated, that most of the people who use "additives" (as I did in my youth) are obsessed with mechanical excellence and it is their attention to detail/quality servicing, where the real advantage is, not the mechanic in a bottle.