Help me understand: UL94 fuel approval vs. compression ratio vs other aspects?

[Immelman]

UL94 is available locally and is now cheaper than 100LL avgas. The more I learn, the more I like it. I'm also kind of over having 100LL exposure to my child, and to my hands, when we go flying. I guess being a parent does that.

Having an IO-360-A1A, it is not approved for that fuel.

My question: What's in an approval? What is safe? Looking at the Lycoming bulletin, I see very similar engines that are approved, i.e. the IO-360-B series. The IO-360-A series in our Mooneys has an 8.7:1 compression ratio, the -B series 8.5:1. Is that the sole difference between engine variants that's allowed their approval of lower octane, or is there more to it?

I am curious if our resident chemists can help me understand how such a small change in compression could lead to bad behavior if running the lower octane fuel?

Put another way: What is the eventual likelihood of our engines being approved for lower octane fuel in the future? Is it an issue of physics or investment in testing?

 

 

Edited February 11, 2022 by Immelman

[dylanac]

I'm no chemist, but my hunch is that they are starting with the lower hanging fruit with regards to certification. It's not a huge difference with regards to compression, but our A1A is further down the list. Also, I was under the impression that none of the injected 360s were approved yet. What is the -B approved in? Hopefully it comes soon, though. I would love to move away from leaded.

[Igor_U]

My understanding is IO-360 A Series (and other 200HP angle valve Lycomings) can't be certified with 94 octane fuel, at least not being rated at full certified power.

IO360 B engine is fuel injected 180HP parallel valve engine and no different then carburetor O360 in M20C, C172 and such... Octane number needed is not all about compression but also internal pressures, timing etc.

[N201MKTurbo]

They will run an instrumented engine on different octane fuels and find out when it detonates. They will build in a cushion of a few octane numbers to find the minimum octane numbers that the engine is safe to use. 
 

The CR, mixture and timing all effect the detonation octane number.

[jaylw314]

There are a ton of IO-360's out there, hopefully they'll come up with something reasonable and easy, like "200 HP limited to 5 minutes continuous" or something like that.  

[kortopates]

There are a ton of IO-360's out there, hopefully they'll come up with something reasonable and easy, like "200 HP limited to 5 minutes continuous" or something like that.  

Neither Lycoming nor Continental are going to change their TCDS for lower octane fuels - that would be hugely expensive and instantly make them ineligible to be installed in their existing airframes. A lose-lose proposition.

the only win-win solution is a certified 100UL fuel. We have it, G100UL, it’s just going to take some time to work it’s way through the approval process.


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[ArtVandelay]

Neither Lycoming nor Continental are going to change their TCDS for lower octane fuels - that would be hugely expensive and instantly make them ineligible to be installed in their existing airframes. A lose-lose proposition.

the only win-win solution is a certified 100UL fuel. We have it, G100UL, it’s just going to take some time to work it’s way through the approval process.

Doesn’t the TCDS specifically mention 100LL, so a STC will be required?

[201Mooniac]

Lycoming has released information about which engines will run on 94UL.  The daya can be found https://www.lycoming.com/sites/default/files/SI1070AB Specified Fuels.pdf.  The TCDS does specify 100LL (at least for the M20J).

[kortopates]

Doesn’t the TCDS specifically mention 100LL, so a STC will be required?

Yes, G100UL got its approved STC back in July 2021, but has a very limited list of approved engines. it’s going to take some time before we see our Mooney engines on the list. But the initial STC approval is a huge step towards the solution we need.

94UL is a non starter unless you’re flying a trainer.


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[Andy95W]

37 minutes ago, kortopates said:

94UL is a non starter unless you’re flying a trainer.

 

You may need to read Lycoming’s SI more carefully.  My M20C’s O-360 is approved for 94UL.  My previous airplane (Piper Aztec) had IO-540s that are also approved for 94UL.  Unless you believe those airplanes are actually trainers, in which case I withdraw my comments. 

[kortopates]

You may need to read Lycoming’s SI more carefully.  My M20C’s O-360 is approved for 94UL.  My previous airplane (Piper Aztec) had IO-540s that are also approved for 94UL.  Unless you believe those airplanes are actually trainers, in which case I withdraw my comments. 

of course not! Pardon my overly broad generalization. But with few exceptions their list contains mostly the set of engines that were approved for 91 octane.
Yet it certainly does include some larger displacement engines i wasn’t aware of including your C. So thanks for pointing that out.


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[Hector]

For engines that are approved (O-360 series) the service instruction says the airframe must also be approved and and the note below applies.

WHEN USING THE UNLEADED FUELS IDENTIFIED IN TABLE 1, LYCOMING OIL ADDITIVE P/N LW-16702, OR AN EQUIVALENT FINISHED PRODUCT SUCH AS AEROSHELL 15W-50, MUST BE USED.


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[Greg Ellis]

Forgive me if I phrase this incorrectly but, at 94 octane, is it true that the power output will be less and thus the miles per gallon of fuel will also be less than the 100LL?  And if so will it be enough to notice…meaning you will go less distance for the same amount of fuel and it be noticeable?

 

[A64Pilot]

There is no significant difference in BSFC with lower Octane fuel, so no difference in consumption, power output should not change either.

Octane does not increase power, it does however allow higher compressions and greater spark advance  which does increase power

Unless timing is changed to accommodate the lower octane, which I’m sure isn’t being called for, but it’s one way to accommodate lower octane fuel.

However some fuel is less dense and that can make a slight difference in consumption, slight.

Your car for instance, if it will run fine on 87 octane regular gas, putting premium in it won’t increase its power or decrease its fuel consumption, Octane in excess of min required does no good, except of course adds cushion to prevent detonation, and we have a rather large cushion with 100 Octane fuel.

Lycoming tests engines for Octane requirement in rather severe conditions, since cyl head temps of 500 are allowed, they test at that cyl head temp etc. So yes it’s my opinion that it’s likely we could get away with 94UL because we don’t run cyl head temps at 500 etc., but the safety cushion woud be much smaller. That day you forget to takeoff with cowl flaps open could be a lot more severe an outcome than if you had 100 Octane.

However my Wife’s CTS-V with its LS6 engine requires premium, it will run fine on 87 Octane because it has a knock sensor, which will turn down timing to prevent detonation, the less spark advance decreases both fuel mileage and power noticeably, if we had modern ignition systems with knock sensors in our aircraft then I’d bet lunch we could run 93 Octane car gas just fine.

After hurricane Katrina only 87 octane fuel was available for awhile so we had no choice.

 

My C-85 in my Cessna 140 will run fine on 87 Octane car gas, actually better than on 100LL, but when it’s running on 100LL there is no difference in power output nor does it burn less fuel

Edited February 12, 2022 by A64Pilot

[N201MKTurbo]

FWIW,

for octane measurements they make a special fuel that is a mixture by (weight or volume or molar, don’t know) of normal heptane and 2-2-4 tetramethylpentane (octane) as the test fuel. Pure octane is 100 octane on most scales.

[N201MKTurbo]

Octane number has no connection to the amount of energy contained in a gallon of fuel. It only relates to how much cylinder pressure the fuel can withstand before detonation. The higher the pressure the more power the engine makes. Anything done to reduce the octane requirement of an engine will require a reduction in cylinder pressure and thus power. Your gas mileage shouldn’t change. The cylinder pressure comes from burning fuel. You reduce power by burning less fuel.

 

Our IO-360s would probably pass the detonation tests with UL94 if we limited our manifold pressures to 27 inches or so. This would only affect those flying out of the short costal airports. It would have no effect for those flying out of airports above 2500 feet or so.

[RobertGary1]

I don’t see why we’re fiddling with 94ul when the only real workable fleet solution is g1000ul. 

[N201MKTurbo]

19 minutes ago, RobertGary1 said:

I don’t see why we’re fiddling with 94ul when the only real workable fleet solution is g1000ul. 

Because as the OP said, He can buy 94UL at his airport, but you can't buy G100UL anywhere.

[philip_g]

18 minutes ago, N201MKTurbo said:

Because as the OP said, He can buy 94UL at his airport, but you can't buy G100UL anywhere.

I totally wouldn't buy the 94 and run it anyway. 

[N201MKTurbo]

28 minutes ago, philip_g said:

I totally wouldn't buy the 94 and run it anyway. 

I don't think anybody was suggesting you should. I thought we were just having a "what if" discussion.

[philip_g]

7 minutes ago, N201MKTurbo said:

I don't think anybody was suggesting you should. I thought we were just having a "what if" discussion.

That's why I really totally super seriously wouldn't do that 

[N201MKTurbo]

6 minutes ago, philip_g said:

That's why I really totally super seriously wouldn't do that 

FWIW,

I once flew my M20F with premium unleaded because of a fuel company screw up. It was in Denver, so not a lot of MP. It flew normally.

[jaylw314]

9 hours ago, Greg Ellis said:

Forgive me if I phrase this incorrectly but, at 94 octane, is it true that the power output will be less and thus the miles per gallon of fuel will also be less than the 100LL?  And if so will it be enough to notice…meaning you will go less distance for the same amount of fuel and it be noticeable?

 

Sort of/not really   Motors that can run 94 octane tend to have lower compression ratios, and thus their BSFC will be slightly less, but that's because of the motor, not the fuel.  If you ran that same motor on 100LL, you'd presumably get identical mileage.

[Immelman]

4 hours ago, philip_g said:

I totally wouldn't buy the 94 and run it anyway. 

Yeah neither would I. If some gets in there by accident, kindly tell us how it went?

But yeah I have to wonder: What is the basis? Has the testing been done, and they found 94 octane was not satisfactory, or has the testing just not been done?

I was not aware of the IO-360-B being the parallel valve engine. But: same displacement, same number of jugs, same timing (I think I read the -B series also ran 25 degees BTDC). Slightly higher compression..... Is that slight boost in compression enough to doom our motors to require higher octane? The valve design?

Just trying to understand the "why".

And I agree a 100UL fuel would be best of all. Where can I buy that, and for what price? Dont think I can.

And I am serious when I say I won't run the unapproved fuel, because I am not a chemist and do not understand the principles involved. I'd love to learn more!

Edited February 12, 2022 by Immelman

[A64Pilot]

I think the comment on why are we messing with 94UL wasn’t for us high compression fuel injected guys, but maybe meant for Aviation in general.

Best answer for that I’d have is that when I was a kid there were often three grades of fuel available, but pretty much always two, 87 and 100. Would be good I think to have a lower priced lower octane fuel for the majority of general aviation that can run it.

Just like cars most don’t need premium and can run the cheaper 87 octane and save money. makes sense to me, maybe assuming the quantity of fuel sold justifies having two tanks, trucks or whatever. It may not, back when Auto fuel STC’s became available, many predicted you could buy it at every airport, but that’s not happened, my guess is there is no money in supplying it or it would be available.

Plus I believe 94UL is literally 100LL without the lead, and many are more trustful of that.

Way back when the University of Tennessee ran a a test with a twin Cessna running one engine on what was then called “Gasahol” with no issues, I’m not sure what if any restrictions or adjustments were made on the engine.

My money is that no testing has been or will be done for 94UL on engines whose TCDS require 100 Octane, doesn’t matter if it would work or not, the TCDS requires 100 and that’s pretty much the end of the story.

My personal belief is that our engines could run it with restrictions, lower cyl head limits, and maybe reduced timing, which could make it say a 180 HP motor and not a 200 or maybe not?

I can tell you that a not insignificant number of people with Experimental aircraft are operating on 93 Octane car gas.

However it should be pointed out that the methods to rate Auto Octane and Aviation Octane are NOT the same.

What would 100LL’s octane be if rated like car gas? I don’t know, does anyone? I think it’s 96 Octane.

Edited February 12, 2022 by A64Pilot