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90 Octane?


NC Bill

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If you are running a 912ULS or a 912i don't go below the required 91 octane. the ethanol is a non issue. Remember, you are downgrading from 100 octane. the 80hp engines can run the lower octanes. I gave a full list of engines and octanes in an earlier post on this thread.

 

But if you must mix here is a rule of thumb. You should not allow the tank to become empty, and then add 50% of lower octane, followed by 50% of higher octane. The fuels may not completely mix immediately, especially if there is a density difference. You may get a slug of low octane that causes severe knock. You should refill when your tank is half full. In general the octane response will be linear for most hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89. Same roughly applies to mixing 100LL and unleaded gas.

 

 

CTLSi

What gives the 80hp engine the ability to run on lower Octane fuel?

 

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Anticept, I have never heard any comment about different densities for gasoline. It would seem unlikely that Rotax would approve the use of 100LL with mogas already in the tank. ??

(The general public doesn't realize that higher octane fuel actually has less explosive power, but is necessary to retard the detonation in high compression engines.)

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Anticept, I have never heard any comment about different densities for gasoline. It would seem unlikely that Rotax would approve the use of 100LL with mogas already in the tank. ??

(The general public doesn't realize that higher octane fuel actually has less explosive power, but is necessary to retard the detonation in high compression engines.)

 

That's fine, because it's all above 91 octane. Mixing a lower than 91 octane with another fuel blend can sometimes create pockets of low octane fuel that can be ingested. When I say sometimes, I mean *extremely rare*, but it's a 20k engine and a 100k plane we are talking about, and I can't see the cost savings and additional hassle being worth anything.

 

I also do not know how well a rotax ULS/i engine can take a lower than 91 rating. I prefer to err on the side of caution and just do what the mfg recommends in this case.

 

As for the less explosive power: there's so many myths revolving around gasoline, I'd like to ask for a source. One thing I know is true, it takes more squeezing to make it deflagrate (gas doesn't technically "explode").

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My local choices are 90 octane with no ethanol, and 93 octane with ethanol. Doing some research leads me to believe that the local 93 blends in Georgia have very low ethanol content, 5% or less. Given this, the CT's excellent tolerance of ethanol, and the hassle of blending, I just run 93 with ethanol and the engine has run like a top.

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It's been mentioned here before that MOGAS is less stable than AVGAS, at least as far as octane goes.

 

So, it may start at 90, but goes down from there.

 

I would not hesitate to top off a plane 1/2 filled with 100LL with 90 octane, ending up with about a 95 octane, ethanol free mix.

 

But, as always, best to err on the side of caution.

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Ethanol does not factor into octane rating. The fuel should state how much Ethanol you are getting, ie. E10 or E20 etc. thats the percent of Ethanol present. Some think the Ethanol should be tested with each batch to make sure you are not getting a higher amount. I dont do that, I simply put in 91E10.

 

I think he means he doesn't like to use ethanol because it accelerates the breakdown of the fuel. Ethanol also has less energy per unit volume than gasoline (1/3rds less), so you have to burn a little more fuel.

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Higher octane burns more slowly and evenly, correct? As I have come to understand it this lowers the power available to an engine that does not need it ( low compression). In a different sense the power does not get used well if you use low octane in an engine that needs higher octane as preignition and hot spots occur in the burning which, of course can lead to engine damage. It's this correct?

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Mogas is not more or less stable than Avgas. Sometimes the Ethanol blended gas can phase separate after LONG periods of non use. But thats months. All fuels can get crud and water. Thats why the gascolator check is part of every checklist regardless of fuel type.

 

Ethanol gas is also tougher on small engines, but only if you try to use months old gas also. Stabil can help that, but its always better to get fresh gas for the lawnmower, weed eater, snow blowers etc.

 

Phase separation is not a matter of time but of saturation. The blended fuel can saturate only so much water before it separates and this can happen just after the moisture is introduced.

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Anticept, I am confused by your comment: "Gasoline octanes can be of various densities, and may not mix." Do you mean the more dense fuel will be found in greater concentration at the bottom of the tank if some of both are put in the tank? Do you mean not mix at all (separate layers)?

 

I find it hard to imagine that any gasoline sold as vehicle fuel would not mix with another gasoline sold as a vehicle fuel. As you noted, "there's so many myths revolving around gasoline, I'd like to ask for a source".

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I think he means he doesn't like to use ethanol because it accelerates the breakdown of the fuel. Ethanol also has less energy per unit volume than gasoline (1/3rds less), so you have to burn a little more fuel.

 

Correct. Here in GA the pumps merely say "fuel may contain up to 10% ethanol", but there is no state mandate that it MUST contain some percentage, as some states have. As a result fuel here may contain 10% or 0% at the same octane number.

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OK, so this is a bit confusing. People on this forum have claimed less stability for mogas, not related to phase separation. Phase separation takes a lot of moisture. I have seen some claim you should throw it out after one month, six weeks, and three months. The truth is out there... somewhere.

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Higher octane burns more slowly and evenly, correct? As I have come to understand it this lowers the power available to an engine that does not need it ( low compression). In a different sense the power does not get used well if you use low octane in an engine that needs higher octane as preignition and hot spots occur in the burning which, of course can lead to engine damage. It's this correct?

 

That is essentially correct. The best power and fuel economy for a given engine will be an octane rating that completely prevents pre-ignition, but no more than that. With aircraft engine we tend to be more conservative than that, because they operate in such a wide range of temperature and density altitude conditions, and the results of detonation are so much more dangerous that losing a tiny bit of power over a fuel which is too resistant to detonation.

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CTLSi: I provided you a several links in a previous thread about ethanol accelerating mogas breakdown. One was from AOPA, which should be considered a reputable source. Ethanol helps boost the octane number, so when it separates from the fuel, your octane number lowers and becomes more unstable.

 

 

Doug G: Gasoline is unstable. After a certain amount of pressure, it will ignite from pressure and heat alone. Higher octanes are formulated with different additives that suppress detonation. Tetraethyllead used to be the big one before it was banned. The downside to higher octane numbers, is the different additives are more expensive. The idea that energy per unit volume or that burn rates change with the octane changes is a huge massive widespread myth. Octane rating refers, and ONLY refers, to the amount of compression a fuel can handle before it self-ignites. Octane ratings are used for more than just gasoline too

 

Even two fuels that have the same octane rating, but from two different refineries, can slightly vary in the total energy stored in the fuel and how it burns. Having a higher octane by itself does not necessarily mean that the fuel burns better, faster, slower, or has less energy, it all matters on how the refinery blends it. The exception is with ethanol fuel. If you add ethanol to boost the octane, the overall energy reduces because ethanol has less energy per unit volume.

 

 

 

FredG: Different blends have different densities. Refineries go through a huge amount of processing to create a homogeneous solution. There are a lot of both heavy and light molecules that go into the formulations. Think of it like milk: if it's not agitated and mixed correctly, the cream separates from the water. Mixing two different octanes can do the same thing, but it's not really that common. In a car, I wouldn't care. In a plane, the extra effort and few pennies of savings isn't worth the tiny increase in risk.

 

 

Here's an article that overviews how gasoline is made, and touches on how straight gasoline (using only iso-octane and heptane) set the standard for performance numbers. Straight gas isn't used, it only set the standard.. It's very difficult to get any specific information about fuel blends from refineries because of trade secrets.

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There is all kinds of chatter about Mogas with E10. The bottom line is the Rotax 912i burns it quite well, and the FD CTLSi is designed to use it. I have used nothing else for 90 hours and the plane runs as advertised - never a cold start, never a ping, never a knock, never a rough idle or cutout.

 

I buy the gas at a cheap, low-cost place for half the price of 100LL on the field. I fill a transfer tank with a pump and filter and pump it into the plane whenever i choose. I keep the truck in the hanger with the plane so fuel is always available.

 

All European aircraft have the special constraints of noise and the use of non leaded gas. Its a good set of goals to have. Our antiquated and slow moving aviation situation for fuels is time-limited. Eventually 100LL will be gone. And replacing it will be unleaded gas with and without Ethanol.

 

The Europeans are even going to E20. Shutter....

 

All the noise regarding Ethanol is moot. Phase separation is due to old gas sitting for long period, sometimes in extremes of temperature and moisture. Its a non issue if you use the gas in as timely way as you would your car. And especially if you have a heated hangar, as I do, so that moisture and temperature extremes are kept away from the craft.

 

If I were not to fly and use the fuel for an extended period (2 months or more), I would drain the fuel. I would also expect to change the oil if the plane sat for that long. And I would even consider changing out the coolant as well. Fluids get nasty when they are not circulated and used as designed. ALL OF THEM.

 

Oh, and when on the road, I use Decalin. I have already mixed 100LL with Mogas and Decalin and it all runs just dandy. But that is rare. If I have to burn 100LL for more than one tank, I simply make sure to put in the amount of Decalin needed with each fueling.

 

So far, this aircraft has performed as advertised. Heck, the plane even survived a non rotax oil filter...

 

 

What is the scientific reasoning and the design specifications that would require the oil and coolant to be changed? Please post the specifics from the manufacturer of the oil and the coolant.

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CTLSi: I wasn't contesting anything other than your statement that you made about fuel stability being unaffected by ethanol. It IS affected, and quite directly.

 

 

mocfly: oils degrade over time and separate. Two months of sitting isn't much to worry about, but oil is always changed at annuals.

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Anticept,

What do they separate into? What empirical data do you have that supports that assertion? Oil is "always changed at annual". What if I change it 5 hours before I drop it off for the annual?

 

 

CTLSi,

My question to you still stands?

 

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Mocfly: That depends on the person doing the annual. Likely they will change it because of liability, and someone has to clean the oil screen (not a normal function of preventative maintenance, I don't think rotax engines have it though).

 

As for oil breakdown: one main cause is oxidation. It is ALWAYS oxidizing, which compromises the lubrication properties.

 

Oil is also an extremely complex blend of chemicals that work best when kept in a homogenous solution. The more I research looking for a reputable source to back up my claim, the more I am thinking this is only a tiny factor in many others that apply. See below.

 

http://www.machinerylubrication.com/Read/475/oil-breakdown

 

http://www.le-international.com/uploads/documents/068_Motor%20Oil%20Degradation.pdf

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What liability would exist if the oil wasn't changed at annual?

 

These articles cover industrial machinery and earthbound motor vehicles. I would contend aviation oil is formulated differently and these articles that are referenced while informative, really do not support your assertion that oil separates or CTSi's assertion that "If I were not to fly and use the fuel for an extended period (2 months or more), I would also expect to change the oil if the plane sat for that long. And I would even consider changing out the coolant as well. Fluids get nasty when they are not circulated and used as designed. ALL OF THEM."

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What liability would exist if the oil wasn't changed at annual?

 

Seriously?

 

I'm going to put something forward to you as a mechanic. You never trust what comes out of a pilot's mouth. I don't know what oil you put in, and I don't need you coming back if you have an accident trying to blame me that I didn't do my job. Oil is extremely critical to engine operation, and I want to make sure my signature that is in your logbook is both a guarantee to you that it's safe, and a guarantee to me that it's been done right. Also, I can't clean an oil screen in most planes or inspect the oil for large particles or discoloration without draining it, and for such an important fluid, I'm not putting it back in because of the increased risk of foreign contamination. I'll do this stuff to my own plane that I only use personally, but the moment someone else uses it, or if it's a customer airplane, my standards of maintenance shoot WAY up.

 

Aviation oil for lycoming and continental engines are natural, because it has superior properties to deal with lead scavenging. Rotax use semi-synthetic (or fully synthetic if you use mobil one, but you CANNOT use 100LL with it!), and are not exempt from the same oil problems as earth vehicles and machinery. All oils break down over time. As I had said, the more I read about oil separating, the less I am concerned with it since this was more of a problem with older, natural oils, which had a lot of additives, whereas synthetic oils are designed from the ground up to resist these problems. I'm more concerned about oil oxidizing, once opened, at this point.

 

I wish I could give you better sources and data. I can only try to find reputable online resources, because the material I have is proprietary, and I don't have a license to distribute any of it.

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