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91UL / UL91 (merged thread)

91UL and plug fouling

i friend of mine has put 250 hours on his o-200 using ul91 and he claims he no longer suffers lead fouling of the plugs.

has anyone else noticed anything similar. i all so wonder if the oil is cleaner as it is no longer loaded with lead does this mean and reduced engine wear

There is no lead in UL91 so there won't be any way to foul the plugs with lead; that's for sure.

As for the other stuff it would be interesting if anybody has done any research... lead itself is a lubricant too.

Administrator
Shoreham EGKA, United Kingdom

One should always operate the engine at or near peak EGT on the ground. Lead fouling should be no issue then with 100LL.

Yes, Lead fouling is very certainly reduced in approved engines while operating on unleaded gasoline. I have over 3500 hours flying unleaded Mogas, and am totally convinced. 2500 of that is on my O-200, the rest on O-470's a carb'd 520, and various smaller Lycomings. Interestingly, the old 80/87 Avgas was nearly always lead free too. Though its standard permitted it to contain lead, I have been told by Avgas experts that it only had this allowance to permit it to be transported in the 100/130 system, where it could pick up a little lead.

The producers and sellers of 100LL are never going to cut into their market for their only product by telling you that operating on Mogas or 91UL is a good thing to do, so you have to consider what they say with a grain of salt. The low compression engines do not require the lead for octane, so they just blow through the lead, which does not foul their plugs. Its a lot of very expensive pollution for no good reason. Leaning helps the engine, but not the environment.....

Home runway, in central Ontario, Canada

Yes, Lead fouling is very certainly reduced in approved engines while operating on unleaded gasoline. I have over 3500 hours flying unleaded Mogas, and am totally convinced.

I think that's ummmm... pretty good data :-))

Running Lycomings and the like on unleaded fuel is apparently still a bit novel in Europe. In the US, these engines were certified (by aircraft specific STC) for use with unleaded auto fuel about 30 years ago and some of them ran on nothing else for a long time.

Interesting data about 80/87 generally having no lead either - I didn't know that. I bought my last red 80/87 in about 2004, apparently it was the last batch in the US. Too bad.

Running Lycomings and the like on unleaded fuel is apparently still a bit novel in Europe. In the US, these engines were certified (by aircraft specific STC) for use with unleaded auto fuel about 30 years ago and some of them ran on nothing else for a long time.

According to their POH, they're not certified for auto fuel as available from gas stations. They are typically certified for 80/87, 100 and 100LL. There have been STCs available for a number of years, also for EASA registered aircraft.

The jerry can do-it-yourself refueling is not common in Europe as it's typically prohibited to transport more than very small quantities (10l) of fuel in a jerry can. However, at least over here most GA airfields offer Mogas.

One issue with auto fuel / Mogas is its ethanol content. It varies from delivery to delivery and it was pretty bad when the new EU law requiring more ethanol appeared. Most Lycoming/Continental STCs limit ethanol to 1% whereas the Rotax fleet can take up to 5%. Nowadays the suppliers seem to have found out where to get low ethanol fuel. I was told that the fuel pipelines cannot take ethanol (bearings) so buying it from a source that gets it from the pipeline does the trick.

My Cessna 172M has been running on Mogas ever since I bought it. The only disadvantage I've noticed are the yellow fuel stains from spilled Mogas.

FWIW, I have had just 2 cases of fouled plugs in 10 years of operation.

Both cases were with fine wire iridium plugs - RHM38S which incidentally I don't use anymore due to the Champion resistor problems described elsewhere here.

One was 10 years ago, following an un-leaned ground taxi. It was cleared by a peak-EGT runup. The other was on the ground at San Sebastian and that was very hard to clear, but it cleared immediately when I restarted the engine about an hour later.

So my own experience is that lead fouling is not an issue with 100LL.

I do my own 50hr checks and always find no lead deposits in the upper plugs, and always find some in the lower plugs, around the base of the insulator, and these have to be carefully dug out. But they are never anywhere near the spark gap.

Administrator
Shoreham EGKA, United Kingdom

what i would really like to see is further stc that allow mogas with 5 or 10% ethanol.

i bet very little needs to be changed. as i know of at least 2 c150 one 172 and one pa28 that just pour the stuff in and i suspect it contains ethanol and it runs fine..

i also know of at least a dozen jodel running continental engines that run on mogas and of all of those one guy tested all the seals /only one swelled/ and then replaced them with ethanol comparable seals.

such an stc for the the common types of aircraft used in flight training would save the industry a fortune.

My understanding is this:

91UL is avgas 100LL without the TEL (lead) additive. So the only risk in using 91UL is in the engine. There is no possible risk to the fuel system. Hence, approvals for 91UL require only the consent of the engine maker. (In practice, I understand from a highly respected US engine shop, any engine with a CR below 9.5:1 is OK on 91UL, so long as the CHT doesn't go crazy e.g. 450F).

Anything with "extra stuff" is potentially an issue with fuel system materials, seals, etc. This means Mogas, and it also means any "avgas" with an octane rating greater than 91UL because there is no way to achieve that without adding "something".

It also means any "100UL" has this issue, in theory anyway. I hope George Braly (GAMI) drops in soon to set me straight

Obviously, getting approval from an engine maker (still in business) is going to be easier than getting approval from them, plus also an airframe maker (often out of business, and even if still in business in most cases not selling that airframe anymore).

From the engineering perspective, one does need to be very careful with fuel system materials. The effects can be very slow. So e.g.

ne guy tested all the seals /only one swelled/

is a very poor test. One does actually have to go right through the whole system and check everything for chemical compatibility. And to be legal, for planes with an ICAO CofA, this needs to be done by somebody with the right authority e.g. an FAA DER who then generates the Approved Data for an STC, which will be specific to the airframe(s) of a specific S/N range that use those materials, or can use different material if they can be changed. E.g. you can change o-rings but you can't easily change fuel tank materials or sealants. Then you can sell that STC

One thing which I'd like to know about is how people actually manage with mogas. In the UK, very few airfields carry it at a pump (I've never seen it) and if you have to use jerry cans then the weight is huge. For example to fill up my TB20 I would need approx 250kg of the stuff. That's completely unmanageable unless you are Geoff Capes (or his modern equivalent) and even then one would need to be able to drive a vehicle to the aircraft, which is not allowed at most airfields. Even at the PA28 kind of level it appears totally unmanageable. Lifting a single 20kg can would do my back in. From speaking to some pilots who use it, they seem to be mostly flying small microlights which need only say 20-30 litres.

Administrator
Shoreham EGKA, United Kingdom

One thing which I'd like to know about is how people actually manage with mogas. In the UK, very few airfields carry it at a pump (I've never seen it) and if you have to use jerry cans then the weight is huge.

Many if not most GA airfields in Germany officially carry Mogas. The price difference here is large, currently at my homebase € 2.57 for 100LL and € 1.88 for Mogas. The Rotax powered fleet is very large these days, they would just not buy fuel if there was no Mogas. The fuel station is the main source of income for most GA airfields.

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