What octane is best, and why

[Bull Mountains]

So basically octane rating has nothing to do with how fast a fuel burns a higher octane fuel can burn faster than a lower octane fuel.

Yes!

 

[Bull Mountains]

I'm not a finished product blender, but I sit near enough to one to know that in our toll refinery (different crude feed every week or two) that getting on-grade gasoline each delivery takes a recipe of multiple components, and that recipe changes often depending on the components in stock as a result of the original crude oil they were extracted from.
Fuel is complicated.

As a refinery guy myself this is spot on.
Gasoline isnt a monolithic product. Its soup with the recipe changing by the day or even hour depending on the lab results.
Also the difference between 87 and 92 is pretty minimal blend wise. 93 just has a splash more alkylate or reformate and that's about it ,with the caveat ethanol isn't being blended in. Ethanol by virtue of it's high octane numbers does allow some poorer quality streams to be blended in and still hit RVP and Octane numbers.

 

[Bull Mountains]

This is octane business is not something you can ‘logic’ your way through, gasolines are made up of 100’s of hydrocarbons, empirical testing of its behaviour must be consulted, combustion kinetics is a beast of a subject, to the point for years there was not a commonly accepted theory to the exact mechanism of knock.

Initially there were two theories, the auto ignition theory, that the end gasses would auto ignite before being combusted by the main flame front, after decompasong to species more apt to combust.

The Detonation theory, that a true detonation/shockwave event would explode through the unburnt fuel charge in the combustion chamber when it pleased.

Then the excess flame speed theory, simply that the fuel burn rate/pressure rise was fast enough to induce knock symptoms, but that no auto ignition or detonation was taking place, I believe this was the third theory, interesting reading, but it was not correct, the generally accepted theory is now that the "end gas" products away from the flame front autoignite when the time/temperature situation they are placed under leads to compositional changes, free radicals and then autoignition.

Flame speed can play a part in octane rating, but it does not correlate across all tested fuels, it is true for some fuels a slower burn rate can reduce knock by limiting peak combustion temperatures, placing less thermal stress on end gasses, conversely slower combustion can induce knock, as the the slower flame front takes longer to reach and consume the end gasses at the periphery of the combustion chamber, leading to the time element of the time/temperature atack on end gas composition winning and initiating Knock, you can read about Hydrogen addition to speed up combustion, reducing knock, through the process of not allowing the end gasses time to autoignite.

You can be a stable fuel that doesn't devolve into the wrong 'species' in the end gasses.
You can burn fast enough to not allow the end gasses to overload.
You can burn slow enough to not thermally overload the end gasses, I question if this is something actually targeted for use in practice though?

Generally we see option 1, be stable under stress, as burn rate does not correlate well to octane rating.
And I know what you are thinking, latent heat of evaporation helps, yes, but propane also has a high octane rating, so it's not mandatory.

Octane rating is a measure of a fuels ability to withstand knock, period.

Fuels can have same auto ignition temp, but different octane rating, same vapour pressure, different octane rating, same flame speed different octane rating.

A quick google for some charts of in cylinder flame front speed measurement shows some interesting figures, they do note show flame speed correlates with octane rating.

Below iso-octane @ ron 100 Vs Ethanol @ ron 109
LINK to source

iso-octane on the left E100-Ethanol on the right.
View attachment 289499

Below another SI in cylinder combustion speed experiment.
LINK to source

View attachment 289505

Think they made a mistake, RON of Ethanol is closer to 109 than 129.





More data, though not from actual internal combustion engines, still relevant though, as combustion speed can be (VERY) roughly simplified as laminer burn speed plus turbulence.


From the 1986 NASA Technical Paper 2596
Velocity Profiles in Laminar Diffusion Flames

Note that Iso-octane and N-hepatne, the actual 100 and zero octane reference points of the RON test are within one percent of each others for laminar flame speed, with 100 octane points between them, other tests may have a larger difference, but this be NASA.

View attachment 289504
View attachment 289506
Think I got carried away.

TLDR

87 might burn faster than 91 pump and operate better in a chainsaw, but that does not mean higher octane = slower burning as a rule.

Best post I have seen on the subject...ever.

 

[Redfin]

Seeing as this is the new model engine thread, check this out. Disregard the son running his suck in the beginning. His dad is an outstanding machinest and engeneer. He designed and built this at his home shop along with the car it fits in.

Where is the car vid?

 

[Egg Shooter]

Where is the car vid?

Not finished yet. I have this....

 

[Egg Shooter]

Where is the car vid?

My bad. You meant the other guys. Lol. On the YouTube. Its a hybrid the v10 car.

 

[jetsam]

I think every topic, if you look into it deeply, has a set of information that is commonly accepted by experts in that field. And then when you look deeper into it, you'll get to a place where nobody knows what the hell is going on because theoretical physicists are still spitballing theories and nobody's invented a way to observe or measure the actual process. And below that is the stuff that the next generation will puzzle over after the stuff we learn proves to be only part of the picture.

It's hard to find a subject that the average guy doesn't think he understands, but the only way to find a subject which is actually understood is to choose a very narrow definition of subject.

If this conflicts with your world view, that's normal. People like simple. But do a practical experiment. Pick the most knowledgeable person you know, pick a question in their field, and ask them why that thing works like it does. Now ask them why the process they just explained works like it does. You'll be very lucky if you can get 5 recursions deep into that conversation, even with a hypothetical someone who knows the entire body of human knowledge- because we really don't understand how anything actually works yet.

This is why I love these threads though- it shows that there are people out there who think about stuff and try stuff (instead of deciding that they know The Truth and never thinking again).

 

[Larry B]

I guess ignorance is bliss. I used to fill my gas can up at the station, pour that into my stuff and turn the key or pull the rope and it makes lots of noise and i use it. But now i have to worry about all kinds of things like flame temperatures, flame front propagation, alcolates, distillates, volatility ratios, hydrocarbon chains. The list is endless. Probably going to lose sleep over this. Life was so much easier when i didn't know to worry about these things. Oh the humanity!

 

[Egg Shooter]

TE="rogue60, post: 1225646, member: 578"]So basically octane rating has nothing to do with how fast a fuel burns a higher octane fuel can burn faster than a lower octane fuel.[/QUOTE]


Looks like it. That probably explains why canned fuels, designed for small engines, are often made in the low to mid 90's octane wise yet don't hinder power. At least in my seat of the pants usage. I wouldn't use cutting wood as a test median because the 1 second or less difference in cut speeds, as seen in the redbull vid, can easily come from variables in the wood and operator. Not intentional either. I'm saying that the great majority of us, if the wood was identical in every way, can't reproduce exact cut times over and over. And to me, a 1/2 a second means nothing anyway. I don't race.

 

[Stihl working Hard]

Now I have a headache.

Thansk

I still can’t believe it’s not locked yet

 

[Bull Mountains]

I think every topic, if you look into it deeply, has a set of information that is commonly accepted by experts in that field. And then when you look deeper into it, you'll get to a place where nobody knows what the hell is going on because theoretical physicists are still spitballing theories and nobody's invented a way to observe or measure the actual process. And below that is the stuff that the next generation will puzzle over after the stuff we learn proves to be only part of the picture.

It's hard to find a subject that the average guy doesn't think he understands, but the only way to find a subject which is actually understood is to choose a very narrow definition of subject.

If this conflicts with your world view, that's normal. People like simple. But do a practical experiment. Pick the most knowledgeable person you know, pick a question in their field, and ask them why that thing works like it does. Now ask them why the process they just explained works like it does. You'll be very lucky if you can get 5 recursions deep into that conversation, even with a hypothetical someone who knows the entire body of human knowledge- because we really don't understand how anything actually works yet.

This is why I love these threads though- it shows that there are people out there who think about stuff and try stuff (instead of deciding that they know The Truth and never thinking again).

And people like to think in absolutes. When your dealing with a highly variable chemical soup like gasoline there are few absolutes.

 

[Bull Mountains]

TE="rogue60, post: 1225646, member: 578"]So basically octane rating has nothing to do with how fast a fuel burns a higher octane fuel can burn faster than a lower octane fuel.

[/QUOTE]Looks to like it. That probably explains why canned fuels, designed for small engines, are often made in the low to mid 90's octane wise yet don't hinder power. At least in my seat of the pants usage. I wouldn't use cutting wood as a test median because the 1 second or less difference in cut speeds, as seen in the redbull vid, can easily come from variables in the wood and operator. Not intentional either. I'm saying that the great majority of us, if the wood was identical in every way, can't reproduce exact cut times over and over. And to me, a 1/2 a second means nothing anyway. I don't race.[/QUOTE]
Uncontrolled variability and sample size renders most of these home spun tests anecdotal at best. But they sure can be fun.

 

[Bull Mountains]

Looks to like it. That probably explains why canned fuels, designed for small engines, are often made in the low to mid 90's octane wise yet don't hinder power. At least in my seat of the pants usage. I wouldn't use cutting wood as a test median because the 1 second or less difference in cut speeds, as seen in the redbull vid, can easily come from variables in the wood and operator. Not intentional either. I'm saying that the great majority of us, if the wood was identical in every way, can't reproduce exact cut times over and over. And to me, a 1/2 a second means nothing anyway. I don't race.

Uncontrolled variability and sample size renders most of these home spun tests anecdotal at best. But they sure can be fun.[/QUOTE]
Canned fuels great. I just can't stomach they price given all they are is straight run alkylate or a blend of alkylate and reformate. That and a chain saw is a piece of simple, cheap equipment all things considered

 

[Bjorn]

Hello. Take a auto tune chainsaw, and fill it With regular gas and put it on a dyno. And measure the Hp. And then you put in the high octane fuel and run it again in the dyno. Just to compare the HP With different fuel.

 

[ddiesel2006]

Hello. Take a auto tune chainsaw, and fill it With regular gas and put it on a dyno. And measure the Hp. And then you put in the high octane fuel and run it again in the dyno. Just to compare the HP With different fuel.

Has anyone actually done this and if so where are the results?

 

[Bull Mountains]

Hello. Take a auto tune chainsaw, and fill it With regular gas and put it on a dyno. And measure the Hp. And then you put in the high octane fuel and run it again in the dyno. Just to compare the HP With different fuel.

Results would be meaningless. All sort of variable uncontrolled and rarely does a dyno give the same results back to back.

 

[Catman]

This octane versus that octane, this oil versus that oil, this chainsaw versus that chainsaw, east coast logger versus west coast logger, this bar versus that bar, this chain versus that chain, so on and so forth. Laughable and very entertaining! Just sayin

 

[Egg Shooter]

Im a low use guy. I buy the VP or Sunoco in 5 gallon cans. Maybe 5 gallons every couple years. Price for me is a non issue. I hate the smell of regular gas in a 2 stroke anymore no matter the oil. In addition, because of its long storage life and my growing vintage saw collection, I like to run it in the saws. My mowers and snow blower gets 90 E-free from the pump.

 

[Wonkydonkey]

I'm not a finished product blender, but I sit near enough to one to know that in our toll refinery (different crude feed every week or two) that getting on-grade gasoline each delivery takes a recipe of multiple components, and that recipe changes often depending on the components in stock as a result of the original crude oil they were extracted from.
Fuel is complicated.

extasec,,, I know where your coming from. Different crudes are of different compositions. And depending on what you want to make... you'd boil and mix different crudes. Even to say you'd boil abit then mix with another bit that’s been boiled .

I remember a little from general chemistry and then. A bit more from chemical chemistry breaking and forming bonds in the petrochemical industry. been a while thou

 

[rogue60]

TE="rogue60, post: 1225646, member: 578"]So basically octane rating has nothing to do with how fast a fuel burns a higher octane fuel can burn faster than a lower octane fuel.

Looks to like it. That probably explains why canned fuels, designed for small engines, are often made in the low to mid 90's octane wise yet don't hinder power. At least in my seat of the pants usage. I wouldn't use cutting wood as a test median because the 1 second or less difference in cut speeds, as seen in the redbull vid, can easily come from variables in the wood and operator. Not intentional either. I'm saying that the great majority of us, if the wood was identical in every way, can't reproduce exact cut times over and over. And to me, a 1/2 a second means nothing anyway. I don't race.[/QUOTE]


I've had a stock ms660 suffer detonation the piston had like melted molten bubbles worse around the squish band area of the piston. I put pics of it up years ago I put it down to off fuel it was low octane to start with.
I don't care about cut times or any of that I struggle in summer with fuel boiling and vaper locking I think it's to do with the ethanol in it.
If I could afford to run canned fuel I'd run that here it's very pricey.
I've tried avgas it works good in my saws dare I say in my hands they feel to make more power back up to full factory spec hp running on it.
I think the problem is the cheap pump fuel here is made for new vehicles not industrial 2T's.