correct fuel for 914 turbo ?

[BONZAI BELL]

Hi guys
Somebody has tried to convince me that we are using the wrong fuel in our gyros , at the altitude we are flying up here in Jhb. most of us are using 95 unleaded. According to this person , ( which sounds like he knows what he is talking about ) we should be using 93 octane. Apparently , 95 octane is a slower burning fuel than 93 , and because of our altitude , we have less oxygen , less dense air , and therefore we need a faster burning fuel for the altitude we are flying in. Down at the coast , 95 octane would be the fuel to use.
I hope i understood him correctly .
Could we get some confirmations from the experts on the forum on this matter.
Regards
Swannie

[OzGyro]

Hi All

Swannie - Maybe this will explain it, copied from a Rotax dealer website

Feeding your Rotax 4-Stroke Aircraft Engine
Fuel | Engine Lubrication | Oil Filters | Cooling Liquid | Spark Plugs

Fuel
Octane rating

The Rotax 912 A/F/UL engines require a minimum octane rating of 87 AKI, commonly referred to as "regular" while the Rotax 912 S/ULS/ULSFR and 914 F/UL require a minimum of 91 AKI ("premium").

Some may have noticed that the Rotax manuals mention ratings of 90 and 95 respectively. These ratings are calculated based on the RON standards used in Europe and are equivalent to a rating of 87 and 91 under the AKI standards used in Canada.

In all cases, we recommend using as high an octane rating as possible, since fuel evaporates and quickly loses its octane rating by osmosis when it lays in your aircraft's fuel tank or in a plastic jug. A "premium" fuel will see its octane rating reduced to unusable levels after as little as three weeks. Fuel with a lower octane rating would obviously have an even shorter usable life.

Too low an octane rating will cause pre-ignition and detonation which can damage the piston ring grooves, skirt and crown.

Where to buy

It is recommended to buy gas at the busier gas stations of the major oil companies, since their tanks are renewed often allowing the fuel to stay fresh and clean.

Aviation Fuels

It is possible but not recommended to use 100LL AVGAS, since the the lead content is like cholesterol to your engine: it will accelerate wear on the valve seats, create deposits in the combustion chamber and sediments in the lubrication system and gearbox. Increased maintenance is necessary to compensate. Unlike "conventional" aircraft engines, lead is absolutely not essential to the proper lubrication and operation of a Rotax 4-stroke aircraft engine. The increased octane rating also has no marked advantage for the operation of your engine.

To be avoided:

"Regular" fuel except if used in the 912 A/F/UL and burned entirely on the day of purchase;
"Premium" fuel which is more than 3 weeks old
AVGAS except when the required automotive fuel is not available


Hope this clears things up a little.

Cheers
Darren

[JvTonder]

2 strokes = 93
4 strokes (912&914) = 95

[BONZAI BELL]

Thanks guys
after i placed this post yesterday , i googled the situation of 93 vs 95 , and somewere it was also mentioned that 95 should be used at the coast and 93 inland. So it is still a little confusing. Any how , i will stick to 95 .
Thanks for your input
Swannie

[Dobbs]

Hi Bonzai Bell, I do not have a 914, nor am I an expert on fuel, however, the 914 is turbo charged, so the amount of air fed into the cylinders at your elevation is the same as it would be at the coast, in fact it is the same up to 10 000' if I have my info right.

I have also never heard that the higher octane is a slower burning fuel, it just has properties which delay the onset of detonation?

[BONZAI BELL]

Hi Dobbs
On the 914 , u hardly use the turbo. you might use it on take off , if hot and and loaded heavy . so i would immagine that at full power , ( not using the turbo) the 914 would basically have the same performance as the 912. So we are back to the altitude problem , where i have heard that u lose approx. 3 hp for every 1000ft altitude. I googled 93 vs 95 octane , and somewhere read that it recomended that 95 should be used at the coast , and 93 to be used inland. ( not sure if this was cars only ) .
Now that you mention "properties which delay the onset of detonation " , this must be the 95 octane. I think what this person tried to explain to me was : that the delay in detonation (of 95 oct.) at our alt. would have the result that a certain amount of unburnt fuel would exit through the exhaust valves , and could cause burnt valves.
Although my instructor , has almost 800 hrs on his gyro (and uses only 95 ) , and has never had a problem. So i might just stick to 95.

[Dobbs]

Hi Bonzai Bell, please don't shoot me down if my info. is wrong, but I have been told that the 914 is essentially the 912 (80 HP) with a turbo attached, you will see that in the capacity, where both the 912 and 914 are 1100 cc and the 912S is 1300, or there abouts. Now going to the RPM vs power dellivered, the 914 is higher than the 912 (80 HP) all along the curve which tells me that you are getting turbo boost at any RPM, and that the only restriction, as with all 9 series engines is max of 5 mins above 5500 rpm.

Appreciate comments if I am wrong.

[Kalahari]

Hi Bonzai Bell, please don't shoot me down if my info. is wrong, but I have been told that the 914 is essentially the 912 (80 HP) with a turbo attached, you will see that in the capacity, where both the 912 and 914 are 1100 cc and the 912S is 1300, or there abouts. Now going to the RPM vs power dellivered, the 914 is higher than the 912 (80 HP) all along the curve which tells me that you are getting turbo boost at any RPM, and that the only restriction, as with all 9 series engines is max of 5 mins above 5500 rpm.

Appreciate comments if I am wrong.

I can not comment on the capacity but agree with the turbo that continuosly provide boost, the turbo get driven by the flow of exhaust gas which in turn drive an impeller that force additional air into the cylinders, the higher the revs, the faster the flow of exhaust gasses and the faster the turbo spin that force more air into the cylinder, a turbo can not be switched of, as long as the engine run, you will have a flow of exh gas and the turbo will spin which provide boost, you however do have a waste gate on the turbo which will open to let some exhaust gases bypass the turbo in order to regulate the pressure in the intake manifold. This is why normal aspirated engines perform much better at sea level than on the rand while turbo models have the same performance at different heights above sea level.

[Dobbs]

Hi Kalahari, I agree with your thoughts, which brings us back to the original discussion point, which is why I believe that 95 octane should be used in the 914 at any airfield elevation, because, at any altutude below 10 000', the engine is essentially experiencing sea level pressures / conditions - any experts to confirm or enlighten

[Gyronaut]

Swannie, these are very valid questions! Well done for picking it up. Most of us actually have no idea.
Kalahari, Dobbs and OzGyro are quite correct.

In simple terms (and I am simple so I will try to explain it best I can)...

Your 914 turbo runs all the time. Fact. Check it out for yourself, if you have a Manifold Air Pressure gauge you will find that before you start the engine, it sits at about 29 inches of mercury (29.92 to be exact). I am talking about at the coast - depends on density altitude - but that is another lengthy discussion. Now start the engine and open the throttle to 100% power (to the stop in a Magni) 100% power is 35 inches of mercury. That means (at the coast) that the turbo is actually pushing 5inHG more than standard atmosphere and you can run at that power all day - 29 inches in the cruise is better because then the turbo is simply idling along. If you 'engage the turbo' it (should) go to 40 inHG (no more - beware) This is in effect 115% power (max 5 minutes) because any longer then your turbo will overheat and seals/gaskets will go etc.

Rotax specifies that you should use a different oil (fully synthetic/semi synthetic) if you are going to run on AVGAS (100LL) for extended periods rather than on the 'unleaded' you usually use. I am told the reason for this is that the lead deposits in AVGAS will be absorbed by the oil. I can't remember which oil for which fuel and don't feel like looking it up late on a Friday night. Ask your AP and if he is worth his salt, he will know. If he doesn't know... find another AP. Mine does.

Look after your engine and it will look after you. Give it the right fuel/oil. Also, more knowledgeable people than I have taught me that it is crucial to warm up and cool down your engine adequately and correctly or it will let you down. Take your time about this.

Fly safe

Len

[BONZAI BELL]

Dankie Len , en al die ander manne.
Alles is stadig maar seker besig om in plek te val. Jammer dat ek jul so gereeld pla met al hierdie vrae. dit maak my slimmer, en hou julle slim manne op jul tone.
Dit word opreg waardeer
Groete
Swannie

[Dobbs]

Hi Gyronaut, one correction, and that is you can only use fully synthetic oil if you are guarenteed not to use avgas - fully synthetic oils have almost no lead absorbing capability.

Shell and Rotax have jointly developed an oil which Rotax recommends, cannot I recall the name, something along the lines of Shell Aerosport 4, but I spoke to Shell head Office in Cape Town recently and the oil is apparently not yet available in SA?

I use Shell advance VSX 4, but I know that there are a number of other high performance 4 stroke motor cycle oils which are just as good.

[OzGyro]

Hi All

Just a little more info in regards to lead concentration levels in oil. In the attached document it shows examples of lead levels in 914 engines, the first grouped resuts are from a 914 engine from the US that has either been run on 100LL or Avgas, the second single example is from a local 914 that has been run solely on 95 Mogas and using Shell VSX4 15W 50 oil. You will notice the dramatic differences in the lead(Pb, highlighted in White) contamination levels between the samples. Another interesting note is also the significantly higher copper (Cu) levels also present in the US samples, this copper contamination can more than likely be attributed to the wearing of the friction plates in the slipper cluthes, thus the reason I guess they suggest shorter oil drain intervals on engines running on Avgas. I hope this helps out.

Cheers

Darren

[Dobbs]

Hi OzGyro, interesting, one question, I see that there are no hours recoreded for the 3rd and 4th examples in the first table, any idea how many hours old those two samples were?

[OzGyro]

Hi Dobbs

I dont know the hours on the other two samples sorry. The sample history works from right to left, the oldest of the samples will be on the right hand side of the table and the newest one on the immediate left,I can only assume they may have been from the initial 100 hours on the motor. I hope this helps.

Cheers
Darren