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100LL engines - going nowhere?

Article by John Zimmerman on AirFacts. Of course, this is US-centric, plus we don’t have the EPA here, but there sure doesn’t seem to be a clear path into the next thirty years.

My guess is that in 20 years either everything will be more or less as it is today, or there will only be electric aircraft (on the two-seat primary training level as well as on the recreational fun-flying level), a few mogas-capable engines/aircraft, plus a few diesel engines below the turoprop segment. Or something in between. But I don’t think there will be a 100LL replacement fuel.

Last Edited by boscomantico at 05 Dec 21:09
Frankfurt (EDFE, EDFC, EDFZ), Germany

The one relatively ‘simple solution’ wild card that I can see is potential STCs for Anti Detonation Water Injection in conjunction with reduced octane unleaded fuel. This would probably allow high compression, larger displacement aircraft engines to operate on 100LL with the lead removed and no other changes. It would only be required at high manifold pressure (takeoff power), which might make it practical for those engines. The rest of the fleet can already run on 100LL with the lead removed – a lot of current engines including both that I own were certified on even lower quality fuel e.g. 80/87 Avgas that hasn’t been available for a couple of decades.

Friends have just finished building a Reno racing Glasair III that’s making about 600 HP from an angle valve Lycoming, turbocharged and running about 80 inches MP. It burns 115/145 octane Avgas but ADI is a key component in preventing detonation and its currently overcooled, IIRC running CHTs around 330 degrees F.

Last Edited by Silvaire at 05 Dec 21:45

Currently perhaps 90% of GA, maybe more (I don’t know what % of the Cirrus scene uses more than 8.5:1 compression, but they sold under 10k planes), can run on 100LL with the lead simply removed i.e. 91UL. And that 10% is shrinking. Well, all of piston GA is shrinking but that’s another story…

However I read somewhere that the planes which cannot use 91UL are buying 30% of the total avgas sales – because they do a lot of flying. That is the dilemma for airport fuel policy… and why TOTAL’s 91UL push in Europe has failed. They tried to fragment the market but the airports know which fuels actually sell. Hence a single fuel is the only solution.

If the whole GA scene was the domain of one big company, they would – to use a current example of big-corporate behaviour – “do a Volkswagen” i.e. play for time and with each year passing the number of customers affected (in the VW case, by the emissions software cheat and its nonfunctioning “fix”) gets smaller and smaller and eventually they can do a (by then small, and generously provided for in their Accounts) settlement to make the issue go away once and for all. Also many owners, like us (we have two), will have had the “fix” reversed so they are ok indefinitely. 15 years from when this started, about 10 from today, the problem will disappear completely. This will happen in GA too, but much more slowly…

In Europe we are probably ok because there is absolutely zero problem manufacturing TEL and avgas generally, according to Warter fuels who reckon the market is a few hundred million € a year.

Given these difficulties, I reckon nothing will change in the next 20-30 years. Well, apart from many of us being dead Avgas is manufactured all over the world. Electric propulsion for GA is not even remotely anywhere near being even on the horizon – unless you just want to train takeoffs and landings in the circuit.

Why can’t someone develop an additive like these which can be poured into the tank during filling up? A lot of “classic” cars use these. I suppose even mixing is the #1 challenge.

That water injection system is interesting, especially this bit: “At least one European manufacturer, Tecnam, will reportedly use ADI in a new twin called the P2012. Powered by Lycoming’s new TEO-540-A1A, Tecnam says the airplane will be operable on mogas”. Did that ever happen? The article was from 2013. The biggest problem I see is the weight: 20kg.

Shoreham EGKA, United Kingdom

In one of the tanks of my io360m1b I use 100 octanes (RON) car gasoline, which currently costs 1.65eur/L. In Italy we have “super” 95 octanes (RON) car gasoline (currently costing 1.5eur/L) and “super plus” 100 octanes car gasoline, which is the minimum (100 RON, 94 AKI) required by Lycoming for the io360m1b (of course I test it for alcool every time, which takes 5 seconds with a kit I got from US, but is always absent in Italy despite regulation allows up to 5%). I’m curious if in other EU Countries it is easy to find “super plus” car gasoline with 100 octanes (RON). I think “super” and “super plus” are legal names in EU law, so should be the same names in all EU Countries.

United Kingdom

I find the 10% of aircraft that can’t use use UL91 representing 30% of avgas used surprising.

I split myself between 3 airfields one of which is pretty large and at every one by far the largest customer for avgas are the flying schools and I would guess they represent 90% (plus) of all avgas sold. And everyone of those aircraft could run on UL91.

I believe the reason why UL91 hasn’t taken off is it’s more expensive than AVGAS. If it was 10 pence a litre cheaper than AVGAS I think the uptake would be much greater.

It’s probably not true on the UK GA airfield scene, where turbo planes are not common and twins are rare for various reasons (fuel cost, maintenance cost (most twins in the UK are in a condition/age which makes them maintenance money black holes), the annual checkride, etc). And yes in the UK most activity is flight training, with nearly everybody chucking it in shortly after they get the PPL

In the USA where all these factors are very different. Despite the huge marketing success in the SEP sphere (and for good reasons) twins are still widely operated and fly a lot for utility. Europe has little utility value for GA, for various reasons.

91UL failed because it is almost impossible to get an airfield to carry two fuels. TOTAL were “giving away” bowsers (worth, what, 50k?) and they still failed. Yes they didn’t deliver the price differential (dumb arrogant marketing IMHO) but I think it would need to be much more than 10p. Most people who go places in reasonably nice planes couldn’t care less about 10p/litre. 50p starts to get attention, and €1 definitely gets attention.

I think the “mogas” issue is the same. One just cannot split the market, and jerry can delivery is unviable (in most situations) for anything that burns significant fuel.

Shoreham EGKA, United Kingdom

In northern Scandinavia we will have a fuel supply issue in the near future. Up north a lot of airports stop with AVGAS and you need to rely on the aeroclubs to keep it there. It is a big concern for us.

UL91 is quite established in Sweden and I would say that almost a majority is it for their sub-180hp engines. And it is cheaper than AVGAS, more or less similar to produce but taxes are a bit less.


Most non-turbo and non-aerobatic-competition-level engines can run 91UL. My IO540-C4D5D can… 250HP.

Shoreham EGKA, United Kingdom

I believe the reason why UL91 hasn’t taken off is it’s more expensive than AVGAS. If it was 10 pence a litre cheaper than AVGAS I think the uptake would be much greater.

That surprises me. in Switzerland it is up to 30 cents per liter cheaper where it is available.

Fuel prices Bressaucourt

LSZH, Switzerland

Peter wrote:

91UL failed because it is almost impossible to get an airfield to carry two fuels.

Seems very odd to me, unless the airfield is bogged down in self inflicted bureaucratic nonsense and/or general negativity. My “other” airfield, ENOP, in the middle of “nowhere” almost from even my point of view. They have MOGAS, 100LL and Jet A1, and it’s all cheaper than anywhere in the European main land. We have several other airfields with the same service. Then again, on the public Avinor airfields, the ones with not much traffic, there will be Jet A1 exclusively unless some locals put something up themselves.

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