What octane is best, and why

[Egg Shooter]

Used to use the same engines back in the mid-70’s.

Ive never flown a plane. Wrecked a helicopter lol. We have a club who's air strip is a mile or so from me. I really should look into that because, ya know, I need another money pit of a hobby.

 

[Egg Shooter]

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.

 

[USMC615]

Ive never flown a plane. Wrecked a helicopter lol. We have a club who's air strip is a mile or so from me. I really should look into that because, ya know, I need another money pit of a hobby.

I still have and fly a handful of 3S and 4S LiPo warbirds.

 

[Larry B]

We derailed this sumbitch but good. Just to get this back to a fuel thread......

"My fuel is better than your fuel"

 

[Bull Mountains]

Nope not abit
I’m not talking making the most HP available. I’m talking a fuel we can all get to put in our chainsaws that will allow us the best power available to us.

And despite what you believe, flame travel is the problem, anti-knock compounds are the cure. If it’s moving fast inside an environment that wasn’t built to handle it, what to do? You must slow it down. Yea, cost you some pony's but you live to run another day. That’s all racing fuel talk and I’m familiar. I used torco in blown vettes. Didn’t need it in the winter, only for summer time. I don’t buy the article in whole to say the least, it certainly represents that fellows opinion and I can see you agree. That’s ok.
But I’m just running chainsaws. Traded the last vette for a pickup. I gave away my last 5 gals of torco.
Made my tailpipes brown.

You've been lead to water..
But ask your self this. What are some of the things done to a two cycle to reduce detonation? All of them involve reducing heat, decreasing combustion time or both.

 

[Egg Shooter]

I still have and fly a handful of 3S and 4S LiPo warbirds.

I may get a drone. Would especially be useful to fly down to check targets at my 400 yard range.

 

[~WBF]

So did you guys come for story time or a fight?

Ok them, more tall TALL tails from the west.

Fist new saw a bought was a 262xp in 1992.
I had 266 and §hit.
The word was ,you need to run them on high octane like 91 or maybe they had 92? That was just before they made it lead free in BC. That came in in '93. I was thinning for a company and camping on the block with everybody. You get what they give you in terms of fuel and mix. He had good mix & good bar oil.
He had a steady 25 people to do 1000 acres (400 hectares) He had an opportunity with gov, ministry of forest (MOF) in his home town to middle a job. They called it a confer job. The power to grant) He does the invitation then manages it then passes it for payment. He pays on completion then waits for his money. Ollie made $7,000 for doing nothing.
I low balled for the reference and left maybe 35% on the table. Still walked out with 14g in 6 weeks and made sure I made nothing off any cutters.

Anyway ..had to tell that story , now just to tell you this story.

I could run my usual 8 tanks on 87 octane.
I tried the Chevron leaded, high octane and in the same hard day was 5 tanks only
If things were not perfect then I would only get 4 1/2 tanks and buddies blowing the truck horn?

Much cheaper to run but it f*ed my 'clock' up.
My production clock, as I knew it. Never used it again.

 

[rogue60]

I remember on our way to the boat ramp as a kid dad would drive through a BP service station and fill up the boat with good ol ZOOM 2T mix right outta the pump. It all went down hill since then.

Yep Australia run on the good old BP Zoom millions of yards tamed every weekend.. Yeah back when oil and fuel was never a consideration or worry it just worked.
The last good fuel I could get for ope was a few year ago now was the purple unleaded.
I'm no fuel scientist but if I didn't know any better I swear the pump fuel I get is close chemically to paint thinners lol
Pump fuel is a never ending evolving product to meet the
EPA same as the new vehicles it's designed to run in people talk about fuel like it's never changed and base opinions on old out of date no longer relevant information.

 

[jetsam]

It’s not a dumb question. In my opinion, you are wasting your money. I don’t know about cleaner. But your equipment will run better on 87 octane.

Not totally related but kinda. In ‘98 I bought a new Jeep Cherokee. Nice truck. I thought so much of it that I started using Premium gas in it. Not long after when accelerating up a hill, the truck would start to misfire. I thought it was a trans problem for some reason but after checking it out and talking with the tech, he advised me to switch back to regular. It fixed it.

I track mileage on every tank on my motorcycle (which is an easy habit to get into because the trip odometer IS the fuel gauge). It gets just about 5MPG more with 87 than it does with 91/92, and that is a big ass difference. Took me years to notice because I always "treated" the bike to the worse, more expensive gas.

Don't see a difference in power. It's an old caurbeurated straight 4.

It would never in have occurred to me to try gas that cost twice as much in a chainsaw!

 

[~WBF]

I track mileage on every tank on my motorcycle (which is an easy habit to get into because the trip odometer IS the fuel gauge). It gets just about 5MPG more with 87 than it does with 91/92, and that is a big ass difference. Took me years to notice because I always "treated" the bike to the worse, more expensive gas.

Don't see a difference in power. It's an old caurbeurated straight 4.

It would never in have occurred to me to try gas that cost twice as much in a chainsaw!

Weird., as I said the old leaded high octane would go forever and a day in my saw. Never tried it since then.

 

[Bjorn]

Hello. I have been running Shell V-power for many years in my saws, and I Also use octane booster in the mix.it Will burn cleaner in the engine and very low carbon build up.

 

[Kiwioilboiler]

Cat cracker gas, olefin feed for the alkylation unit and light cycle oil for the ultra low sulfur diesel unit.

Nice.
We have HDSs, HCU, BDU, HMU and Platformer to supplement our CDU.

 

[Kiwioilboiler]

Being that we mix oil in with the fuel octane is subjective? After all you "can" run white camp gas in your saw and its 50 octane and very clean.
My major concern is heat that may be generated from lower octane fuel or carbon deposits through hot spots in the cylinder.
I generally use 90-96 e-free gas with a stabilizer and 40:1 (heavy towards 32:1) .
I was tinkering around with a 1973 318cid and an f-code Paxton blower once and researched" Water injection " I made my own with an old desiel fuel pump and some ss tubing. The point was to make pump gas act like race gas ,aka burn cooler and atomize better .
Obviously adding water to fuel is not helping the octane but it certainly helps with knock and ping when making compression!
I have fuel books all the way back to the 50s and 60s on theory and design and I have spoken with fuel designers in reseach laboratories (coal slurry jet fuel)
A high octane is better burning for lower temperature whether you need it or not but most additives that make the biggest advances aren't available anymore. I ran a push mower on benzene for a summer and boy did that sumabitch cut grass!

I could be wrong chief but it's been my understanding that higher octane fuel will burn slower, therefore increasing the chance carbon will be left behind as opposed to lower octane which will be burnt completely up.
Source. Don Weeds, European Marine. Optimax outboard guru.

 

[Maintenance Chief]

The slower burn is more complete. Think about it this way coal burns completely once lit , cardboard burns fast .
Higher octane fuel will burn more completely due to all the molecules in the structure meeting the reaction, like a chain event. Fast burning fuel burns fast but somethings get left behind in the speedy reaction.
Most of the good stuff in gas/petrol evaporates at room temperature through so how long it takes you to use that fuel has alot to do with its quality.
Nitrous oxide is a super power additive because it allows most of the fuel molecules to join up with an oxygen molecule during the chain event they normally wouldn't have time to , if that makes sense?
Being that the fuel burn is a timed event every ignition of lower octane fuel is leaving some of the unburned fuel behind building up and thats where deposits and pre ignition happen in a four stroke creating hot spots.
Im not a scientist and this is not my area of expertise so please do your own research, but when we start modifying saws for performance we should be thinking about the piston getting halfway through the stroke only to have fuel deposits from a prior event ignite and fight the centrifugal force of the piston.
A Pro Mac 10-10 has like 7:1 compression so it'll burn/run on anything short of piss .

 

[drf256]

https://www.sunocoracefuels.com/tech-article/beyond-octane

The discrepancies we are having in the discussion are that the generalities are correct, but as usual exceptions do apply.

In general, lower octane fuels burn faster because they have smaller hydrocarbon chains. Any fuel will burn at a certain heat level because the energy will cause the molecular bonds to break and oxidize. We compress fuel/air mix to add heat and get it to the point where the molecule becomes unstable. Then, the spark adds the last bit of energy needed to make the reaction get past the point where the reaction occurs.

But we also get into densities of hydrocarbons, specific properties, weights, etc...

There will always be exceptions to the rule, but in general a fuel rated at a resistance to detonation similar to Octane (100%=100 octane rating) will have a slower burn rate. The slower burn rate may result in more power (from more controlled and homogenous combustion) or less power.

Interesting tidbit from Wikipedia. We need to consider that the evaporation/atomization of the fuel into the air causes an endothermic reaction which cools the engine and charge. The lower the octane fuel, the cooler the engine should be. The charge causes a significant cooling on a chainsaw engine. This may be part of what lower octane fuels help chainsaw engine’s performance.



“Higher octane ratings correlate to higher activation energies: the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.

Because octane is a measured and/or calculated rating of the fuel's ability to resist autoignition, the higher the octane of the fuel, the harder that fuel is to ignite and the more heat is required to ignite it. The result is that a hotter ignition spark is required for ignition. Creating a hotter spark requires more energy from the ignition system, which in turn increases the parasitic electrical load on the engine. The spark also must begin earlier in order to generate sufficient heat at the proper time for precise ignition. As octane, ignition spark energy, and the need for precise timing increase, the engine becomes more difficult to "tune" and keep "in tune". The resulting sub-optimal spark energy and timing can cause major engine problems, from a simple "miss" to uncontrolled detonation and catastrophic engine failure.

The other rarely-discussed reality with high-octane fuels associated with "high performance" is that as octane increases, the specific gravity and energy content of the fuel per unit of weight are reduced. The net result is that to make a given amount of power, more high-octane fuel must be burned in the engine. Lighter and "thinner" fuel also has a lower specific heat, so the practice of running an engine "rich" to use excess fuel to aid in cooling requires richer and richer mixtures as octane increases.

Higher-octane, lower-energy-dense "thinner" fuels often contain alcohol compounds incompatible with the stock fuel system components, which also makes them hygroscopic. They also evaporate away much more easily than heavier, lower-octane fuel which leads to more accumulated contaminants in the fuel system. Its typically the hydrochloric acids that form due to that water[citation needed] and the compounds in the fuel that have the most detrimental effects on the engine fuel system components, as such acids corrode many metals used in gasoline fuel systems.

During the compression stroke of an internal combustion engine, the temperature of the air-fuel mix rises as it is compressed, in accordance with the ideal gas law. Higher compression ratios necessarily add parasitic load to the engine, and are only necessary if the engine is being specifically designed to run on high-octane fuel. Aircraft engines run at relatively low speeds and are "undersquare". They run best on lower-octane, slower-burning fuels that require less heat and a lower compression ratio for optimum vaporization and uniform fuel-air mixing, with the ignition spark coming as late as possible in order to extend the production of cylinder pressure and torque as far down the power stroke as possible. The main reason for using high-octane fuel in air-cooled engines is that it is more easily vaporized in a cold carburetor and engine and absorbs less intake air heat which greatly reduces the tendency for carburetor icing to occur.

With their reduced densities and weight per volume of fuel, the other obvious benefit is that an aircraft with any given volume of fuel in the tanks is automatically lighter. And since many airplanes are flown only occasionally and may sit unused for weeks or months, the lighter fuels tend to evaporate away and leave behind fewer deposits such as "varnish"[clarification needed]. Aircraft also typically have dual "redundant" ignition systems which are nearly impossible to tune and time to produce identical ignition timing so using a lighter fuel that's less prone to autoignition is a wise "insurance policy". For the same reasons, those lighter fuels which are better solvents are much less likely to cause any "varnish" or other fouling on the "backup" spark plugs.[citation needed]

Because of the high cost of unleaded, high-octane avgas, and possible increased range before refueling, some general aviation pilots attempt to save money by tuning their fuel-air mixtures and ignition timing to run "lean of peak"[clarification needed]. Additionally, the decreased air density at higher altitudes (such as Colorado) and temperatures (as in summer) requires leaning (reduction in amount of fuel per volume or mass of air) for the most power (crucial for takeoff). In almost all general aviation piston engines, the fuel mixture is directly controlled by the pilot, via a knob and cable or lever similar to (and next to) the throttle control. Leaning must be done carefully, as some combinations of fuel mixture and throttle position (that produce the highest EGT) can cause detonation and/or pre-ignition, in the worst case destroying the engine within seconds.[citation needed] Pilots are taught in primary training to avoid settings that produce the highest exhaust gas temperatures, and run the engine either "rich of peak" (more fuel than can be burned with the available air) or "lean of peak" (less fuel, leaving some oxygen in the exhaust) as either will keep the fuel-air mixture from detonating prematurely.[citation needed]”

Oil Thread 2.0

 

[Sawrain]

This 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 not 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.


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



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.



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.

 

[Maintenance Chief]

No body quotes books anymore , Haha.

 

[Kiwioilboiler]

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.

 

[rogue60]

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.

 

[Mastermind]

Now I have a headache.

Thansk