Technical Fuel Question

[nipper1]

I read in the paper today (http://news.independent.co.uk/environment/article325237.ece) that UK fuel suppliers are to be directed by the UK government to include at least 5% biofuel as a constituent part of all Mogas supplied at UK pumps. No date is given for the change.

Given that ethanol is the most likely additive and most STCs and other Mogas approvals prohibit the use of Ethanol-containing fuels what are we Mogas aviators going to do? 100LL is supposed to be on the way out so that does not look like a long-term option.

I’m also interested to understand why ethanol is a problem (especially in light of the fact that there are plenty of test-aircraft running 100% ethanol) and other ethanol fuel mixes. (http://ezinearticles.com/?Flying-Aircraft-on-Ethanol&id=25803 and http://www.agairupdate.com/aau/articles/2005/Feb2005.html)


I know about the issue of seals and hoses (both resolvable at a price), corrosion (possibly resolvable at a very high price) and water absorbtion (presumably manageable with revised fuel handling systems).

Simply put, why can a £5,000 car run on 10% ethanol Mogas when a £500,000 aircraft can’t?

Thoughts and suggestions from the technically savvy please.

[Skycop]

Because a £5000 car was designed very recently, in the light of future or existing legislation, it is more likely to be designed to be compatible than a £500,000 aircraft designed thirty years ago, when there was no need for an ethanol based fuel.

The issue of comparitive vapour pressures of petroleum based fuel and ethanol might also be a factor. Take alcohol fuel to altitude and it increases the likelihood of a fuel system vapour lock, aready a possibility with existing MOGAS in comparison to AVGAS.

[Rod1]

OK so not compatible with 30 year old aircraft designed to run on Avgas. What about a Rotax 912 which was designed from the outset to run on Mogas and has a continuous fuel loop in the fuel system to remove the vapour lock problem?

Rod1

[Mariner9]

Vapour pressure isn't the issue, water absorption is.

The DERD 2485 (Avgas) spec requires the fuel to pass a water microseparometer test. Basically, the fuel has to allow water to "drop out", whereas Ethanol would absorb water. Therefore any fuel containing more than trace quantities of alcohols would not meet current DERD 2485 Avgas specs. All you certified aircraft types can sleep easy (for now) :ok:

However, there are no such requirements in the EN228 (motor gasoline) specs. This is potentially a problem therefore for any aircraft currently running on Mogas. My 912ULS Rotax POH specifically prohibits running on fuel containing alcohols.

I anticipate all manner of AD's in the coming months :{ :{

Edited to add:

ROD1, we'll probaly have to run on Avgas in the short term, but expect we'll be fitting suitably adapted gascolators in due course.

Edited again to correct the above:

It's actually my aircraft POH that restricts alcohol in fuels, not the Rotax manual, which merely "cautions" against it.

[nipper1]

Thanks guys, but not too much help. I'm beginning to wonder if this is a regulations issue, not a technical issue.

For what its worth Rod, a far as I can tell most of the 30 year old engines would be quite happy running on some kind of 80 octane fuel - that is certainly what they were designed for. 100LL has only been foisted upon most of us because a few high-powered turbo engines and warbirds need a high octane fuel and the low demand means it is only ecconomic to transport, store and supply one grade of Avgas.

Vapour pressure isn't an issue (in fact Ethanol based fuels need to have something added -usualy a petroleum distilate - to raise the vapour pressure)

Water absorbtion could well be an interesting issue, but if my car and Mariner's Rotax can run on alcohol-mix without having problems, I'm still not clear why my VW powered Nipper or an A65 can't run on it.

Once again, a request for real solidly based technical answers.

Thanks

[Mariner9]

Firstly minor correction nipper, my Rotax can run on alcohol based fuels in theory (though cautioned against), but my a/c POH definitely prevents it, so its irrelevant.

Secondly, having consulted my one of my chemist colleagues, a slightly more solidly-based technical answer to your question as requested:


Vapour pressure: Not an issue. Ethanol has a higher vapour pressure than some avgas/mogas components, and lower than others. The vapour pressure is simply controlled in the blending process (using a very complex formula backed up by trial analysis). We dont expect vapour pressure specs to change.

Water absorption: Major issue. Alcohols absorb water readily, but the amount of water they can hold in suspension varies greatly with temperature. Therefore, a water drain test on the ground may reveal nothing, but after the fuel has cooled after a couple of hours in the cruise, water would drop out, with obvious results.

This could be handled by incorporation of automatic water drains in tanks/gascolators or more simply by repositioning of fuel tank suctions (though that would obviously lead to a higher quantity of unusable fuel)

Unless aircraft are widely retrofitted with such systems, we really cant see the MSEP spec being dropped from Avgas specs (and my colleage sits on the Institute of Energy panel that advises on such matters)

[Rod1]

The POH for my MCR-01 - for fuel specification – see Rotax 912 manual.

The engine (912ULS) manual does not prevent you using Mogas with some alcohol content. Any thoughts on the legality of doing so?

Rod1