912ULS, Using RON 98 Query

I am from NZ and have used 98 RON occasionally. I did not notice any difference.

I suppose 98 will maintain a "over 95" rating longer than standard 95 so for longer term storage etc. I have heard on people starting off with 98 or 100LL if they are flying somewhere where only 91 is available the logic being that topping up with 91 on top of 98 will still maintain about 95 octane.

Generally speaking having higher octane does not help performance unless you can "boost" the engine.

Famously a shipment of high Octane fuel arrived in England from the US just in time for the battle of Britain that allowed a higher boost pressure and gave the hurricanes and Spits a significant power boost at a great time ! The biggest benefit was a higher climb rate to get up to fighting altitude quicker !

Hi Glenn and Alan

First, the RR engines used in the fighters went into a constant development from the early 1930s to even after the war.  They started with just below 1000 HP, ran on a carburetor (the Germans used fuel injection) and it was a relatively small engine compared to the Messersmith and Folk Wolf aircraft.  Most of the RR engines came from the USA, made under permission by Packard to take advantage of the mass production techniques of the auto industry.  (Ford refused because Henry Ford did not want to do it, he had a big problem with doing it for political and personal reasons) The addition of supercharging and other power improvements were only possible with the introduction of Avgas with higher octane, that much is true  We need to understand that the fuel however is a solution to detonation that is created by the higher pressures of the engine.  

So in simple terms power is a creation of the design by pressure and RPM.  Once you reach the level that allows full power without detonation then adding more octane cant increase power.  There however is a bit of a silver lining to using a better fuel (higher octane).   If the promised octane level is not really present in the fuel you were sold then the higher grade may protect better against detonation.  

Just to be clear, without expensive testing equipment like a refinery has there is no way to test octane.  This is why it is always best to buy fuel from a reputable supplier and fresh.  (no fuel that smells like a skunk is fresh) Also we can't really tell what the octane is when you blend fuel.  It is not something that you can depend on just by mixing volumes and guessing at, If that is your desired method however here is the formula, but remember you cant really state this is for sure because the fuel you are using is suspect in the first place.

( [ % Fuel A ] x [ Octane of Fuel A ] ) + ( [ % Fuel B ] x [ Octane of Fuel B ] ) = Octane of Mixture

one last thing....Avgas is not blended on an RON (ROZ) scale.  It is actually tested to MON (motor octane number) standards.  The test engine is run at 900 RPM rather than 600 RPM.  There are a lot of other items also but that is something for hte fuel suppliers to verify.  

Check it out online, there are a lot of good web pages to tell you about the different tests. 

Cheers

G’day Alan,

Fortunately there is guidance about this for the 912ULS in SL-912-016R2, particularly for fixed or ground adjustable props.

If you can plot your position on the graph it should inform you of your options. I use 95 for my 912ULS with a ground adjustable prop. It’s pitched to keep me within the 95RON boundary on the graph and provide good climb performance at the expense of cruise speed, if I increased prop pitch I’d need to be using 98.

Fresh fuel is key I think in any case, according to the info I have read on the subject, the fuel we endure in Oz is of poor quantity by international standards, particularly the Sulfur content and I’ve also been lead to believe that some aromatics used to help boost the octane rating to 98 are also the first to degrade with time.

Interestingly, there is a note re mixing different fuel types to use the lower RON number, Rotax being typically conservative I think.

https://rotax.my.salesforcesites.com/DocumentsSearch/sfc/servlet.

shepherd/version/download/0681H00000FTeDYQA1?asPdf=false

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 !!!!

https://www.bp.com/content/dam/bp/country-sites/en_au/australia/home/products-services/fuels/opa-lfactsheet-storagehandling.pdf

My two bob’s worth in summary and assuming you have a fixed or ground adjustable prop, if you are on or below the 95RON line you have the choice and using the higher octane will not hurt. If you are above the 95 line, you need to use 98 and ensure it’s fresh or use 100LL Avgas.

Cheers, Des.

Agree with all of the above. I would add:

I have been using 98 RON for about 15 years - in NSW 95 RON is becoming increasingly hard to find. I understand that this is an Australia wide rationalisation, by fuel companies, of product offering . I have never had fuel quality related problem.

I am sure there is a measurable performance difference between 98 & 95 RON (if you have the test equipment) but subjectively I have never noticed any change and this includes the very rare times, when on a trip away, I refueled with AvGas, making a 98/AvGas shandy.

I will repeat the advice from above: ULP is not subject to the very high QA of Avgas & may containe contaminants such as traces of other fuels/H2O/ foreign matter The chances having a problem, will be minimised by purchase from a supplier with a high fuel turn over. I add to this; Use a good quality filter funnel, when refueling, you will be amazed at what it catches/prevents from entering your aircraft fuel system.

98 RON ages/degrades faster than 95 RON and Avgas, when stored in a container that is open to atmosphere (aircraft fuel tank). age this characteristic by not refueling after the last flight (unless flying the next day), rather refuel immediately before the next flight. Adding a significant quantity of fresh 98 RON, to aged fuel, will restore almost all of its qualities. 

Note: 98 RON can be stored in a sealed/airtight container for 6 months or more - the container should be filled to 75% or better and stored in a relativly cool place.

On ethanol blended fuel with a 95 or higher RON: Usually called E10 (10% Ethanol) - I have never imbibed, however I am aware that Rotax allow its use in (some of) their engines -  this does not mean that the rest of the aircraft's fuel system is compatible with E10. If you are considering E10 seek advice from your aircraft manufacturer BEFORE using it. 

Interesting information but not relevant the the BoB,

I do not believe any of the American Merlins made under licence by Packard were produced before the end of 1941 and were initially supplied to US forces.

to Quote Wikipedia re the Battle and US 100 octane fuel..

With 100 octane fuel the supercharger of the Merlin III engine could be "boosted" to +12 lbs/sq.in., producing 1,310 hp (977 kW) at 3,000 rpm at 9,000 feet (2,743 m) with a time limit of five minutes.^[40] This increased power substantially improved the rate of climb, especially at low to medium altitudes, and increased the top speed by 25–34 mph up to 10,000 feet.^{[33][24][N 2]} During the Battle of France and over Dunkirk RAF Hurricanes and Spitfires were able to use the emergency boost.^[41][42] "In the first half of 1940 the RAF transferred all Hurricane and Spitfire squadrons to 100 octane fuel."^[43]

So to me the biggest contribution you Yanks made to the Battle was the 100 Octane fuel. Although the 11 Pilots were of course gratefully appreciated !!

I do not want to get into an argument about who made the best Merlin the Brits or the US !! That is a bit like that great line from the Right stuff "Is it there German rocket engineers...No Our German rocket engineers are better than their German rocket engineers !!"

The biggest Contribution my Country made to the BoB was definitely Pilots (we were third after UK and Polish Pilots). Infact that was a NZ Government policy pushed after the first WW we were too rural and far away to help with Production but had a good education system and deliberately decided to produce Pilots...

Then again perhaps Keith Park head of 11 group was the best Kiwi contribution ??

This has not stopped of course My Best man a Kiwi Pilot ended up in the UK flying Harriers in the 1990's