95 or 98 Ron fuel?

price difference to me between 95 and 98 is 9c, is it worthwhile? Does the extra octane mean you have more net energy and therefore use less of it? In the real world we probably just get an engine that produces more power and gets off the ground fractionally quicker and climbs better? More thinking about cruising consumption. All things equal is it worth it?

 

It’s hard to notice a 5% difference in figures if going from 95 to Avgas at 100 but anybody have any comparison fuel burn figures to substantiate?

 

 

Wouldn't it depend on what the minimum octane rating for your engine is ?

 

 

Most Rotax are can run 95, question I guess is; is there any value that is returned to the purchaser by running a higher octane fuel?

 

Do people that get their Cessna stc rated to run mogas have a poorer fuel economy per litre because there is net less energy in mogas?

 

 

OCTANE NUMBER:

 

The octane number of a gasoline has little to do with how fast it burns or how much power the engine will make. Octane number is the resistance to detonation.....

 

read the full article here ==>> Flame Speed, Octane Number and Horsepower

 

I have been told that in Australia, 98 RON has a slower flame front than 95 RON, thus putting less "thump" onto the piston, but still producing the same power.

 

I use 98 RON in my Jab 2200, but have no proof of which is better - YMMV

 

 

98 becomes 91 in short order in unpressurised containers, ever noticed your car fuel tank is pressureised to prevent this, not many aircraft tanks are, so unless you purchase & use in same week your using 91 anyway.

 

 

Th

 

98 becomes 91 in short order in unpressurised containers, ever noticed your car fuel tank is pressureised to prevent this, not many aircraft tanks are, so unless you purchase & use in same week your using 91 anyway.

This doesn't seem to be supported by technical articles from the oil companies. In fact in an article I read there are instances where 98RON actually went up in octain over a given time period although by a minute amount. However overall all fuels deteriorate or change composition over time

 

 

Way things are going in WA, 95 will be a thing of the past. A number of servos in my area now only offer 91 or 98.

 

 

OCTANE NUMBER:The octane number of a gasoline has little to do with how much power the engine will make. Octane number is the resistance to detonation.....

The sooner people forget this misinformation that Octane Number has anything to do with the energy of combustion and hence the power developed by an engine, the better.

 

Octane Number is simply a means of comparing samples of fuel with each other by comparing the resistance to detonation of each sample with a standard fuel. The standard fuel is a mixture of Heptane and Octane where the percentages of Heptane and Octane are known.

 

Thus 94 "octane" fuel has the same anti-knock response as a mixture of 94% pure octane and 6% pure heptan.

 

 

Octane is an anti knock index .The rate of flame travel is milliseconds different which has little significance until detonation gets involved. The SG of the fuel has more "effect" on energy in it.Nev

 

 

All makes interesting reading thanks everyone. How significant is it that it appears the highest compression ratio of commonly used ra engines appears to be Rotax 912? Lowest seems to be O-320. Should this be significant in the choice of octane?

 

 

Cooler running engines (Liquid cooled ) are less critical than air cooled engines which allow head temps over 200 C as normal. Both Comp ratio and head/ piston temps come into it.. Nev

 

 

The higher compression ratio engines are just the result of normal engine development over time. Don't forget that the air-cooled aero engines are virtually the same as stationary engines used to power farm pumps and the like. They develop their power by having large diameter cylinders and corresponding large diameter pistons. Engines like the Rotax have smaller diameter cylinders, so to get the same or better power production, the engines need to spin faster, and to do that a higher compression ratio is needed. Along with a higher compression ratio comes the risk of detonation, so the need for high octane fuels arises.

 

Remember way back when we used 80/87 in small Lycomings and Continentals? It seems that once the power rises above 120 HP the compression ratio gets to 8.5:1 and from there on up, 100LL is required. Below 8.5:1, 80 octane is OK.

 

https://www.lycoming.com/sites/default/files/SSP-110-1%20Certificated%20Engines.pdf

 

 

All makes interesting reading thanks everyone. How significant is it that it appears the highest compression ratio of commonly used ra engines appears to be Rotax 912? Lowest seems to be O-320. Should this be significant in the choice of octane?

100hp Rotax requires minimum of 95 octane. It has somewhere around 10:1 compression.

80 hp Rotax requires a minimum of 91 octane. It has around 8:1 compression.....

 

Remember these have water cooled heads, so combution chamber temps may be more stable than pure air cooled engines such as the 320....

 

I use 98 because if there is any degrading of the fuel, I'm still above 95.

 

I do decant fuel from the aircraft into sealed cans if not flying for a few weeks...

 

 

I have posted this before somewhere but the attached article from BP is a good insight into the deterioration of fuel in vehicle tanks. Basically whatever you decide to use, make sure it complies with the engine manufacturers specifications and then follow the advice in the document as to longevity. You don't have to throw away petrol that has been in tanks for a while but you do need to add some fresh fuel with it.

 

petrol-life-vehicle-tanks.pdf

 

petrol-life-vehicle-tanks.pdf

 

petrol-life-vehicle-tanks.pdf

Octane determination in Piston Engines

I read somewhere that 95 RON may be on the way out - my local Mobil has just gone down this rout. Previously had 91 & 95 (& something called E10 that they sell very little of). 95 has now been replaced by 98 RON ($1.60/L).

 

On another note: many have loyalty to their perceived better brand (BP is often mentioned) - any of you learned persons got a view on this??

 

 

There was some debate on a thread here a few years ago & the consensus was that BP 95 was the best. Our local BP no longer sells 95 & has replaced it with an ethanol blend that is 94 RON. I used to use BP 95 but now use 98 (not BP). I always top up with some fresh if I have not flown for a couple of weeks. 98 will keep for quite a while in sealed jerry cans but deteriorates quite quickly when stored in a vented petrol tank as the light aromatic components evaporate off first.

 

 

So what do you do if you've been on a flight, land with tanks half full and know that you won't be flying for a few weeks? Do you siphon your tanks back out into sealed jerry cans? Or just top up with fresh fuel next time you go?

 

Would it be a good idea to top up the fuel tanks after you land and put non-ventilated caps on your tanks - effectively turning them into sealed jerry cans?

 

 

I now use BP 98 as my local stopped selling 95. I keep it in jerry cans and fill tanks before flight.

 

 

I drain my tanks into cans if not flying for a few weeks.

 

I'm lucky in that it is easy to do in my aircraft.

 

I would definately not seal them. Variations in temp and pressure could damage them.

 

They are designed light weight and not to contain any pressures....just vented to atmosphere....

 

 

IF it's premixed don't use it after any significant time has elapsed. Nev

 

 

...Would it be a good idea to top up the fuel tanks after you land and put non-ventilated caps on your tanks - effectively turning them into sealed jerry cans?

Good idea Marty, but that's just adding one more item you could overlook during preflights inspection...with disastrous results. Your sealed tanks are likely to deliver fuel until you've warmed up and then construct the supply at some critical phase of take off.