LeSving wrote:
In Norway I guess also elsewhere, but you have to investigate/ask. This “E” marking is simply a play for the gallery, and in every sense of the words. “E5” means “up to” 5% ethanol for instance. This is all EU regulations. The fuel resellers themselves are bound by other (EU) regulations. They are bound by selling X% of the fuel as biofuel, any biofuel. I don’t remember what this X is, but the important part is this X% relates to the total of all fuel they sell. They typically sell lots of diesel with biodiesel for instance. Shell includes biogasoline. Biogasoline is a drop in replacement for gasoline (chemically identical). It’s not ethanol, but counts as biofuel (obviously) for the resellers.Another thing is that 98 octane counts for less than 5% of all the fuel a tThiypical reseller sells. They mostly sell diesel and 95 octane. This means that 98 octane must be stored on average for a much longer time, and its’ very stupid to mix it with ethanol if you don’t need to, for storage reasons. 98 is also marketed as “premium” fuel, and is more expensive. The result is that mixing ethanol into 98 octane is an expense (and a risk) they don’t need to pay, so they don’t do it. Both Esso and Shell in Norway has made it clear they don’t mix ethanol into 98 octane, and have no plans doing so. It would be surprising to me if they did this elsewhere.
This concurs with what I’ve heard from a club in Sweden which had a MOGAS STC on a Cessna 172. The fuel company (don’t know which one) couldn’t guarantee that the fuel was ethanol-free, so the club tested it at every delivery to their tank and it always was.
What I have heard for years is 91UL/96UL is NOT good for Rotaxes. I have previously just dismissed it at nonsense, mainly because Rotax themselves say this fuel is OK. However, this also depends on what the octane rating actually is in the fuel you fill the tank with. According to ULPower, they have measured this, and found the octane rating way below specs. 100LL and MOGAS 98 (as well as fresh 95) is always on spec.
Well… Don’t confuse UL91 and 91/96UL. They are completely different fuels, manufactured according to different standards. 91/96UL is used extensively in Sweden with both Rotaxes and Lycosaurs and I’ve never heard even rumours that it would be bad for the engines.
Airborne_Again wrote:
What’s your problem, LeSving?
Sometimes you write very “encrypted”. It’s not easy to decipher what exactly you mean or say. This or the opposite ?
The Rotax manual is very explicit regarding this “burping”:
It is essential to build up compression in the combustion chamber. Maintain the pressure for a few seconds to let the gas flow via the piston rings into the crankcase.
Sounds simple enough perhaps, but is it? hmm. Most people, if they have read the manual, have only read, or at least remember, or have been told to:
turn the propeller slowly by hand in the direction of engine rotation
Now, if you read only the last part here, what most people do, is to turn the propeller using one hand pushing the trailing edge of the propeller, slow or fast, makes no difference. What happens then is the engine will jump over the compression stroke fast because the moment you are over dead center, the compressed air will push the propeller out of your hand. The force immediately changes direction. You either have to use both hand, or grab the propeller real hard around both the leading and trailing edge. It’s surprising how difficult this is to do for many people, especially using only one hand. Yes, the propeller must be rotated, but the only important thing is:
The whole idea is to build up air pressure in the crankcase. That air pushes the oil up into the tank. When changing oil, you have to do something similar. Then you have to crank the propeller 20 turns. This time it’s the oil pump that does the work to circulate oil through the engine. Now you can flick like a madman, it doesn’t matter 
You can immediately see if a person has read and understood the manual simply by watching how they “burp”.
Don’t confuse UL91 and 91/96UL. They are completely different fuels, manufactured according to different standards
I don’t get that. Surely 91UL and UL91 is the same thing. 96UL is the Sweden-specific fuel.
We burp the engine on the Super Guépard before flight. First you take the oil filler cap off then turn the prop one blade at a time. You need to use both hands as turning it with one hand needs a lot of strength. To turn more than one blade at a time as in a complete turn you would need to be a candidate for the world’s strongest man contest. After turning a few blades you will hear a glug glug glug noise. Some suggest you turn a bit more until you hear a second glug glug.
Only then should you check the oil. Every Rotax ULM pilot here is taught this as far as I know. In discussion with Rotax engineers I was under the impression that this has more to do with oil going into the constant speed prop gearing, otherwise you would get a false oil level reading.
The POH for the Super Guépard recommends the use of unleaded 95 which on petrol pumps is IIRC marked E10. The same fuel is used on the vast majority of Rotax ULMs here. The POH also says that 100LL can be used but if in regular use the oil needs changing twice as often. It mentions that 100LL can cause castle like lumps of carbon.on the.valves of piston heads.
These castle like formations can continue to smolder after detonation causing detonation at the wrong timing as soon as the fuel mix is introduced and.before the piston reaches the position where the spark plug is supposed to burn the fuel. This constant burning of the fuel independent of the spark can lead the engine to vibrate very badly and can eventually lead to engine damage.
LeSving wrote:
Sometimes you write very “encrypted”. It’s not easy to decipher what exactly you mean or say. This or the opposite ?
I’ll try to be less cryptic…
Peter wrote:
I don’t get that. Surely 91UL and UL91 is the same thing. 96UL is the Sweden-specific fuel.
There is no 91UL. There is UL91 (the fuel sold by e.g. BP) and 91/96UL (the fuel sold by Hjelmco). These are no arbitrary labels – it is what the fuel companies (and EASA and Lycoming) call them.
There is (or was) supposedly a 91/98 UL also, but I’ve never actually seen it being sold anywhere.
There is also 91UL = UL91 for consistency with 100LL, in common usage. There is no LL100.
Peter wrote:
There is also 91UL = UL91 for consistency with 100LL, in common usage
That may be, but it is still the case that “91UL” and 91/96UL are different fuels, so any tests or rumours that there are issues with “91UL” does not mean that 91/96UL has the same issues.
It’s interesting to read the various grades of factually incorrect info published by airframe suppliers and supposedly knowledgeable people, as posted by participants here for reference. That tells me there are people involved who know little about gasoline engines or Bing constant vacuum carbs.
I’d guess that the biggest factor that might’ve changed with carburated Rotaxes in recent times is the more common use of alcohol-laden fuel of any variety. Alcohol leans out the mixture unless the carbs are rejetted to suit, a process which takes only a little work but which probably has legal issues for planes in some regulatory regimes. If the fuel has enough alcohol and if you’re running it hard and hot without re-jetting, it is at least a plausible idea to me that it could cause reliability issues.
Also if the existing carb jets are slightly blocked through the use of E10 or whatever, that would make things worse. It’s been my sad experience to clean a lot of carbs so-afflicted although the biggest problem is with small diameter pilot jets that don’t contribute to fuel flow at high power.
TCP additive is useful to minimize lead deposits when using 100LL on engines that are susceptible to lead-based deposits.
To add a data point: in the US the Tecnam P2006T twin is increasingly used as MEP trainer (I did my CPL/ME in one) and AFAICT all run on 100LL. I am not aware of any problems with the engines (Rotax 912 S3).
Btw, there is an alternative method for the ‘burping’: run the engines at idle for one minute, then check. As per Tecnam POH.
