Fuel stabilizers and octane loss

[smokechase II]

I was told that gas in a small container or chain saw gas tank can lose up to an octane a day.

1) Is this true?
2) Just how effective are the stabilizers in the mix oil?

Thanks for any help.

 

[Lakeside53]

I had heard it's 1% per week... but.... it still goes down.

For what it's worth, Stihl advises that it's a waste of time putting stabilizers in the gas as it's in their (and many other) oils. Hasn't stopped me for loading up on Stabil for my Winter gas stockpile, and then using it in saws the next spring and summer.

 

[SteveH]

No chemist here, but I can tell you if I run my 036 Pro with gas that's been sitting around a couple months, it is more difficult to start. I have to dump out any fuel in the tank and replace with fresh, starts right up. Of course, both Stihl manuals these days [at least mine] and dealers tell you to run fresh gas. My dealer says nothing older than 30 days. Whether that's an octane problem, I can't say. With my old Echo and my older Stihl, never had a problem with this. Maybe I was lucky then.

 

[woodshop]

I had heard it's 1% per week... but.... it still goes down.

wow, 1% per week... so if for some reason one of your lesser used saws didn't get fired up in 6 months, the gas in its tank would be 25 octane less than when put in, or about 70 octane. My guess would be that it does go down, but maybe it's not linear, or at some point it bottoms out at a certain octane? Probably effected my many other factors, temperature, exact formula of the gas (winter or summer or maybe alcohol mixed in some parts of country). Do stabilizers like Stabil and the stuff they put in mix oil help retard this octane loss?

I too add Stabil to my gas can, and thus to my mix during the winter when it doesn't get used as much, even though the Stihl oil I use has it already in it.

 

[Four Paws]

The reason that manufacturers recommend using fresh gas is oil can separate from the fuel over time...if you are using a GOOD, high quality oil, you should not have any problems. In fact, you are more likely to have problems with the substandard, poor quality fuel that you pay $$$ for from the pump. Auto fuel quality varries greatly from station to station - often unpredictable, especially at the discount stations. I believe I read somewhere that Amaco has the most consistent quality of pump gas. I personally burn 100LL (100 octane low lead) aviation fuel in all my 2 stroke machines. Yeah, it is $1 more per gallon, and I have to drive to my local commuter airport (about 7 minutes out of my way) to get it, but it is manufactured to a MUCH higher standard - all this means is it is far more consistent. The lead also acts as a lubricant and there are additives to cool the burn. Be sure not to use oxygenated fuel (ethanol) as it will create a lean condition that will fool you into thinking your jetting is spot-on - and you will likely melt your saw down unless you are an experienced tuner. Also, do not use castor oils - they fall out of suspension, especially when it is cold outside. In a sealed container, stored in the shade, 'premium' pump gas can last over 3 months. Just a little advice - hope it helps.

 

[Monkeyhanger]

Hi,

after reading this thread I started wondering if "octane loss" is actually correct. Since high octane fuel is much harder to ignite than low octane fuel then, logically, the fuel would ignite more easily the longer it is left standing if octane loss was taking place!

So, logically, it can't be anything to do with "octane loss" that makes old fuel less eager to burn. So what does cause it? Well I think it maybe just some of the chemically lighter parts of Petrol/Benzin/Gasoline that are evaporating off leaving the heavier, harder to ignite, parts behind. This would explain why Alkylate fuel isn't bothered by long storage as it does not consist of such a petroleum mix that make up normal filling station fuel.

Any other ideas on this anyone?

Bye

 

[bwalker]

after reading this thread I started wondering if "octane loss" is actually correct. Since high octane fuel is much harder to ignite than low octane fuel then, logically, the fuel would ignite more easily the longer it is left standing if octane loss was taking place!

You logic is flawed. High octane gas is no harder to ignite.

 

[Monkeyhanger]

You logic is flawed. High octane gas is no harder to ignite.

hi bwalker,

as I remember it the octane value is a measure of resistance to pre-ignition or detonation - that is the fuel air mix deciding to burn before a spark comes along. Obviously a good spark is not going to be much impressed by a high octane rating. Also carbon buildup and other hot spots won't cause premature ignition in a high octane fuel. That is what I meant by being harder to ignite. I didn't mean that the fuel wouldn't burn if you held a blowtorch up to it!

A high octane fuel will always ignite with a good spark whereas a low octane fuel can ignite without any spark at all. Bearing that in mind my logic should actually be quite correct about high octane fuel being more difficult to ignite.

Anyway, I still think it has to be a problem with the more volatile and lighter parts of the fuel evaporating that causes starting problems old fuel. If fuel had only 70 octane and you put it in your saw and started it you would likely need to go out and buy a new piston as the pre-ignition would have destroyed your old one. As that does not seem to happen when you do get old fuel to run then that also supports the idea that octane can't have anything to do with the problem.

Still, I am certain that octane is not the cause of this problem and that it is probably the loss of the lighter gas components. Or it maybe something else that I haven't thought of yet...

Any suggestions?

Bye

 

[RaisedByWolves]

My guess would be that it does go down, but maybe it's not linear, or at some point it bottoms out at a certain octane?

This is what I was thinking.


Ive had equip stored with gas in it fire right up after sitting for years and others not liking month old gas. I would be inclined to think the sealing propertys fo the vessel would play a big part.


Monkey, Mabye your confused by the fact that higher octane fuels are harder to atomize?

 

[timberwolf]

Monkey, Mabye your confused by the fact that higher octane fuels are harder to atomize?

Where is that from?

How about acetone, Xylene or toluene?

Atomization has to do with surface tension, not octane rating. Vapor Pressure and Boiling Point would factor in as well.

 

[bwalker]

as I remember it the octane value is a measure of resistance to pre-ignition or detonation - that is the fuel air mix deciding to burn before a spark comes along.

Octane is a mesure ofa fuel resistance to detonation, not to ignition. Detonation occurs after the plug has fired.
It is a mistake to equate high octane with slower flame speeds , lower flame temp, etc.
The high octane components of gasoline are often the volitile ones that boil off during storage. Fuel stabiliser will not prevent this. What fuel a stabiliser does is prevent oxidizdation and the formation of gums/varnish .

 

[Monkeyhanger]

Monkey, Mabye your confused by the fact that higher octane fuels are harder to atomize?

Hi,

no there is no confusion on my part. High octane fuels ARE more resistant to ignition, that is what allows you to build high compression engines to get more power. Don't confuse harder to ignite with harder to burn. I have never heard anything about high octane fuel being harder to atomize so I can't comment on that.

I am just using simple logic and a basic understanding of organic chemistry (admittedly from a long, long time ago...) to work this out so far. The simple logic being:

low octane fuel ignites without a spark.

and

high octane fuel needs a spark (preferably a good strong spark) to ignite it, remember that even a weak spark can cause ignition problems.

therefore - low octane fuel ist easier to ignite than high octane fuel, which is another way of saying - high octane fuel is harder to ignite than low octane fuel. Nobody can argue against that logic, can they?

I am also not saying that fuel does not lose some of its octane rating when it is stored ( poorly stored ), but that I don't believe that the octane loss is causing old fuel to become ignition "resistant". You must remember that the octane rating only measures a fuels resistance to pre-ignition/spontaneous ignition/detonation (call it what you will) and not its ability to burn or its energy content.

Since a lower octane would make the fuel easier to ignite (if the octane sinks low enough you won't even need a spark plug anymore), there must be some other factor involved. I reckon it must be a more volatile part of the fuel mix that is missing in old fuel that causes the problem.

I'm still open to suggestions.

Bye

 

[Monkeyhanger]

The high octane components of gasoline are often the volitile ones that boil off during storage. Fuel stabiliser will not prevent this. What fuel a stabiliser does is prevent oxidizdation and the formation of gums/varnish .

Hi Bwalker,

that is interesting, that would mean that I am pretty close with my guesswork so far. If the high octane parts are also the most volatile then that would explain the starting difficulties. I'm still not happy with the octane part yet though, seems to be more of a side effect that doesn't matter if the fuel won't ignite anyway.

I'm starting to get quite interested in this now, looks like I'll be doing a bit of research until I'm happy with the "cause and effect" of this fuel storage problem.

Thanks bwalker!

Bye

 

[Lakeside53]

Octane is a measure of fuel resistance to detonation, not to ignition. Detonation occurs after the plug has fired.
It is a mistake to equate high octane with slower flame speeds , lower flame temp, etc.
The high octane components of gasoline are often the volitile ones that boil off during storage. Fuel stabiliser will not prevent this. What fuel a stabiliser does is prevent oxidizdation and the formation of gums/varnish .

Ben : when a fuel "oxidizes", what is the result?

 

[timberwolf]

There is nothing that can be added to gas to stop the evaporation of the higher vapor pressure components, so all that can be done is to boost the level of these chemicals.


I would bet a big part of the stabil mix is Toluene AKA methylbenzene, phenylmethane, or aromatic hydrocarbons (all just elusive ways to lable the product warnings). Toluene is high octane and has a mid range vapor pressure when compared to other gas components so would tend to stay in (Can make up to 25-30 % of gas anyway priemium having more), also cheap.

They might also find iso-octane and maybe a little acetone, both of wich would evaporate fairly quickly out of stored gas.

They likely also add in some corrosion inhibitors and anti-oxidants to help protect the fuel system.

But do the cost benifit of adding gas sabilizer and you may be better off just buying a fresh gal every couple months. Though it might have application where quantities sit for a few months and then must be relied on.

 

[Monkeyhanger]

Hi,

I've just been looking at the octane ratings of various hydrocarbons used in fuel and it seems that the fuel density (and I assume its volatility - I don't know for sure) are unrelated:


substance : octane : density

n-heptane : 0 : 0.684
n-pentane : 62 : 0.626
2-methylhexane : 40 : 0.679
2,2,3-trimethylbutane : 112 : 0.690
1,2,4-trimethylbenzene :148 : 0.889
methylcyclohexane : 104 : 0.770
toluene : 124 : 0.867
3-ethyltoluene : 162 : 0.865
2-methylbutene-2 : 176 : 0.662

As you can see the lightest in my list has an octane of 62 and the second lightest has 176! I also found out that gasoline consists of roughly 65% iso-parafins and aromatics with octane ratings anywhere from 40 up to 170.

Combining that with the assumption that the lighter parts are evaporating means that there may not necessarily be a big loss in the octane rating. It will make a difference to the willingness of the fuel to bring your engine to life though. Seems to depend a lot upon what the fuel consists of and which hydrocarbons were used to mix up the fuel.

I think I've had enough for today, this is turning out to be more complicated than I had hoped. I never did enjoy organic chemistry anyway...

Bye

 

[manual]

Loss of Octane?

the question I have is what do the fuel companys do to keep the octane Rating up while the fuel is in storage? from what I have read. 1% loss in a week. I would never get 98% anywhere.

 

[Lakeside53]

the question I have is what do the fuel companys do to keep the octane Rating up while the fuel is in storage? from what I have read. 1% loss in a week. I would never get 98% anywhere.

Good question, but for consumer gas, I'm not so sure they do... it moves fast. 100LL for av gas is designed to be stored.

I suspect it has a lot to do with "how" it's stored, and where. Gas stations typically have underground tanks (cool) that are hopefully sealed well!

 

[Monkeyhanger]

the question I have is what do the fuel companys do to keep the octane Rating up while the fuel is in storage?

Hi

I would say that they add more of the heavier stuff with a high octane rating such as the 3-ethyltoluene I listed above. It has a high density and a high octane rating of 162.

I don't know anything at all about the cost factors that may be involved in this though.

Bye

 

[bwalker]

Ben : when a fuel "oxidizes", what is the result?

Tars and gums AKA varnish.
[QUOTEHigh octane fuels ARE more resistant to ignition][/QUOTE]
This is not correct. Octane is a measure of a fuels resistnce to detonation, not ignition. Detonation happens well after the mixture is ignited by the spark plug firing.