Octane fuel degradation

[Guest Fred]

Howdy all.Anyone out there have a formula on 2 stroke 98 octane fuel drop rate per week/month?.Have'nt taken the

 

girl out flying for a month,and I have read that you drop down 2 octane levels just by adding 2 stroke oil.She starts

 

first pull after the fuel is 1 month old,but I would not want her running under 91,and I don't want to get to the pinging stage either.Any helpful imfo would be great.

 

 

[dazza 38]

I dunno but my Husqvarna chainsaw may not be started for 12 months with 12 month old 2 stroke fuel in her and she starts and runs like a dream.

 

 

[horsefeathers]

Try this BP document - its a bit surprising re the octane level, and for 2 stroke users, makes you think twice before flying with old fuel.

 

http://www.bp.com/retail/liveassets/bp_internet/retail_new/retail_new_australia/STAGING/local_assets/downloads_pdfs/0_999/Petrol_Life_in_vehicle_tanksv4.pdf

 

In summary it says:

 

Loss of light components – impact on octane

 

The light components in petrol are lost first as the petrol sits in the fuel tanks. These

 

components provide valuable octane benefits under high revving conditions such as

 

cold start acceleration and the loss of these components can result in detonation and

 

pre ignition at high speed resulting in piston damage.

 

The remaining components that have not evaporated are high octane and octane can

 

actually increase with time but this octane is not available for high revving

 

conditions.

 

 

[facthunter]

Don't fly with old fuel. Mix fresh every time if it's premix.. It makes a beaut weedkiller though. Nev

 

 

[Guest Fred]

Try this BP document - its a bit surprising re the octane level, and for 2 stroke users, makes you think twice before flying with old fuel.http://www.bp.com/retail/liveassets/bp_internet/retail_new/retail_new_australia/STAGING/local_assets/downloads_pdfs/0_999/Petrol_Life_in_vehicle_tanksv4.pdf

 

In summary it says:

 

Loss of light components – impact on octane

 

The light components in petrol are lost first as the petrol sits in the fuel tanks. These

 

components provide valuable octane benefits under high revving conditions such as

 

cold start acceleration and the loss of these components can result in detonation and

 

pre ignition at high speed resulting in piston damage.

 

The remaining components that have not evaporated are high octane and octane can

 

actually increase with time but this octane is not available for high revving

 

conditions.

Good food for thought,very good reading,thanks.

 

 

[kgwilson]

Premium fuel will degrade quicker than basic 91. It is mainly the additives or aromatics that evaporate first reducing the octane rating. As Nev says old fuel makes great weedkiller. It also gets rid of European wasps if you pour some in their underground nest. They don't even get angry. It must sort of drug them first.

 

 

[coljones]

If petrol is left in an airtight drum will it lose the lighter aromatics as there is no where for the fuel to evaporate to except go back into fuel?

 

If petrol is left in a vented tank it probably will lose the lighter aromatics.

 

 

[Vev]

If petrol is left in an airtight drum will it lose the lighter aromatics as there is no where for the fuel to evaporate to except go back into fuel?.

I would still be mindful of oxidation degradation ... Fresh is always best!

 

Cheers

 

Vev

 

 

[turboplanner]

This has got beyond a joke; we are being screwed by the oil companies.

 

I first found out about the issue as a spectator when the owner of a chainsaw was paying out about the cost of cleaning out his chainsaw and the service technician was trying to explain that he needed to put fresh duel in each time he used it, and never to use fuel older than a month. I thought it was a bs excuse from the service outlet.

 

Then I came unstuck with poor starting and rough running on one of mine, fixed immediately by some new fuel.

 

In discussions I was advised that fuel life can be as short as a fortnight, and fuel should be burned right down to zero in cars.

 

This also seemed like pie in the sky, and totally impractical on a farm with multiple engines, some of which are only started once a year.

 

I had a lot of experience at draining methanol from the race car at 1 am after racing to avoid the white powder blockages a couple of week later, but this sounded ridiculous.

 

Then the Briggs and Stratton motor in my rotary hoe, as reliable as the day is long refused to start.

 

Don't tell anyone but this simple little carby got the better of me, and I couldn't get the engine to start.

 

Took it to the local B&S dealer who explained that they had a constant stream of Hondas particularly which gummed up with the stale fuel, and although they cleaned out every orifice in the carby, then engines were only satisfactory 10% of the time, and the only real fix was a new carby at $90.00

 

Having been reassured there were no secret techniques with the carby I took it home and thoroughly cleaned it out, then ran MP Thinners through it, but still couldn't get the engine

 

to run so I put the carby in a saucepan and boiled the bastard for half an hour*. I've been rewarded by the engine running with full choke for about a minute, so it looks like the dealer was right.

 

So I'm going to have an engine start day once a month and invite all the local school kids to the festival.

 

Seriously, although this sounds draconian I'd recommend in an aircraft where you aren't going to use it for three or four weeks to drain the carby and tank(s) and shandy the fuel into the car.

 

In the meantime how do we get the oil companies to tell us what they've done?

 

* Warning, even though boiling in a saucepan is a sterile operation, wives always find out you did it.

 

 

[David Isaac]

Ha ha Tubz, in relation to wives finding out, You are so right ... They find out from the guilty vibes we give off. That damn female intuition. LOL

 

BTW, I think the ideal thing is to run the engine dry each time and don't leave any fuel in the tank. You are right about the fuel manufacturers, one can't help but think the fuel has been designed to go stale.

 

Recently I have taken to shutting down the Auster by turning the fuel tap off and letting the engine shutdown on starvation (I run premium 98 ULP in the Auster). It does two things, gets all fuel out of the carb and ensures that the fuel is always on at start because if it ain't it won't start and there is no risk of starting and moving off with the fuel valve off. The only problem is what do I do with the fuel in the tank. Drain it and take it home for woof wood?

 

 

[Guest Fred]

Ahh all these comments of fresh 2stroke fuel has changed me.I now love the smell of fresh 2 stroke fuel in the morning, smells like victory.

 

 

[kgwilson]

I accept all of the comments, warnings and experiences of others but I have not personally experienced this phenomena myself at all.

 

I used to own various 2 & 4 stroke engines on various bits of equipment when I had a 20 acre block. My big 85cc Husqvarna 380 chainsaw new in 1984 was used mainly for felling trees & cutting large rounds off felled trees for firewood & was used annually for 21 years & sold in perfect operating condition. It usually sat for 11 months with fuel in the tank & always started 2nd pull. Same with the little Homelite but it got a bit more regular use & the Stihl scrub cutter, idle in the winter. The lawnmowers, both ride on & walk behind with 4 stroke B&S & Tecumseh motors were unused for 4 months or so in the Autumn/Winter & it was only the ride on battery that had to be charged before starting in Spring with the same fuel it had in the tank in Autumn. Never had a problem & I still have the walk behind mower, now around 20 years old.

 

Was petrol better 5 years or more years ago? I only ever used 91 unleaded. Ethanol was not an option then.

 

 

[facthunter]

It not only degrades it goes right OFF. (stinks) I don't know why this is but avtur gets microbial activity.( as an example). The rotten little "B"s survive minus 50 to over plus 50. They add something to the fuel to stop it. ( The oil companies) Perhaps something similar is happening to petrol?. Your biggest worry is the loss of octane rating, ( for a quick engine termination). Nev

 

 

[horsefeathers]

.... Your biggest worry is the loss of octane rating, ( for a quick engine termination). Nev

It seems from the BP documentation, that Octane actually INCREASES while petrol is stored, due to the evaporation of the lightweight volatiles over time.

 

Quoted from BP:

 

The remaining components that have not evaporated are high octane and octane can actually increase with time,

 

but this octane is not available for high revving conditions.

 

 

Also, the weight of the fuel increases, and this may affect the fuel / two stroke oil mix.

 

And remember, we are not flying behind brushcutters or chainsaws that have a very different operational life to a two stroke aircraft engine, which is just running flat out most of the time.

 

Old fuel does not provide the high speed "protection" that fresh fuel provides.

 

So, for an aircraft two stroke engine, use fresh fuel, and don't mix the oil until you are ready to use it.

 

And don't go flying behind your chainsaw.

 

 

[turboplanner]

Was petrol better 5 years or more years ago? I only ever used 91 unleaded. Ethanol was not an option then.

Yes it was; I only heard the conversation I was referring to about two or three years ago.

 

What you are saying about your experience was my memory too. We would think nothing of going out to a tractor left in the bush for a couple of years with a freshly charged battery expecting to work it that day,

 

Times are way different now.

 

 

[Vev]

It not only degrades it goes right OFF. (stinks) I don't know why this is but avtur gets microbial activity.( as an example). The rotten little "B"s survive minus 50 to over plus 50. They add something to the fuel to stop it. ( The oil companies) Perhaps something similar is happening to petrol?. Your biggest worry is the loss of octane rating, ( for a quick engine termination). Nev

http://www.bp.com/retail/liveassets/bp_internet/retail_new/retail_new_australia/STAGING/local_assets/downloads_pdfs/0_999/Microbes_in_Fuel.pdf

 

You can get microbes in petrol but this is very infrequent ... it happens often with diesel and kero/Jet under the right conditions.

 

Fuels are much more stable these days, however the volatility varies (deliberately) by region and time of year ... buying fuel in one season i.e. winter and holding it over into summer can see the light ends flash off more quickly as the temperature increases ... opposite when you buy in summer and it goes into cooler months the lighter ends remain liquid for much longer.

 

Avgas is far more less volatile as it is 100% isooctane with virtually no aromatics to flash off ... not very popular in the sniffing fraternities.

 

As I said before, fresh is best

 

Cheers

 

Vev

 

 

[Gnarly Gnu]

There is a fuel stabilizer additive called Sta-bil, I've heard it is very good & recommended by some generator manufacturers but it seems expensive.

 

 

[503]

I accept all of the comments, warnings and experiences of others but I have not personally experienced this phenomena myself at all.I used to own various 2 & 4 stroke engines on various bits of equipment when I had a 20 acre block. My big 85cc Husqvarna 380 chainsaw new in 1984 was used mainly for felling trees & cutting large rounds off felled trees for firewood & was used annually for 21 years & sold in perfect operating condition. It usually sat for 11 months with fuel in the tank & always started 2nd pull. Same with the little Homelite but it got a bit more regular use & the Stihl scrub cutter, idle in the winter. The lawnmowers, both ride on & walk behind with 4 stroke B&S & Tecumseh motors were unused for 4 months or so in the Autumn/Winter & it was only the ride on battery that had to be charged before starting in Spring with the same fuel it had in the tank in Autumn. Never had a problem & I still have the walk behind mower, now around 20 years old.

 

.......agree with this the old honda motor seems to start right up after 18 months,still like the rotax fuel to be reasonably fresh though

[M61A1]

* Warning, even though boiling in a saucepan is a sterile operation, wives always find out you did it.

I found that if you do that sort of thing often enough(and ignore the whining) they get used to it, I use the oven a lot.....mine even asks what I'm cooking now (out of interest) whether its an aluminium cylinder head being preheated or hot curing some composites.

 

 

[turboplanner]

The Briggs and Stratton had what appeared to be a brown wax residue in the very fine pickup filter. It was passing no fuel at all, and if you put it to your mouth and applied suction is would hold the suction. After soaking it in MP (Lacquer) thinners overnight the screen passed air OK but the engine wouldn't fire at all even though I'd cleaned every orifice. After boiling the carby for 30 minutes I've got the engine to fire up and run for a short time on full choke.

 

So I'm of the opinion there's a wax like substance which has not been refined out of the petrol.

 

 

[bushcaddy105]

A relevant observation:- I use Avgas most of the time, as at both home base or en route it is much easier to pull up at the pump and swipe my card. The odd splash or dribble on the polycarbonate windscreen has no effect whatsoever.

 

When using PULP at a country airstrip the wind blew a few drops across the top of the screen where it curves up to the wing, and the screen INSTANTLY cracked and/or crazed along the splash lines. Happened to be Shell PULP for what it is worth.

 

There are obviously some potent aromatics in Mogas that aren't used in Avgas. I will gladly stick with Avgas, despite the price premium.

 

 

[Doug Evans]

I have use a mix of avgas and bp 98 for years with out any problem in both my ligthwings with a 582 when I can get avgas I try and use it because my engines run a lot cooler egt and much more response from the throttle better starting and a much cleaner engine inside .

 

They say that the plugs may fowl but I have never had a problem ,

 

As for premix 25-0229 my first Ligthwing was a premix engine and I had her running a on a mix avgas & 98 for the 11 years with no problem ,

 

 

[turboplanner]

We all did that Doug, this is a very recent issue.

 

 

[Downunder]

Aren't we supposed to keep tanks full to prevent condensation?

 

Anyway......I've taken to keeping mininum fuel in the aircraft (98oct) and putting it in the car after a few weeks if not used.

 

The car does NOT like it at all......and fuel economy drops off substantially. Not sur if this is because of the quality of the fuel or the fact I normally run 91 in the car..

 

 

[David Isaac]

Keeping the tanks full applied to aircraft generally when all we used was Avgas. Particularly with the bladder tank types like Cessnas and yes it kept condensation to a minimum. Avgas does not go 'off' like ULP. However, i am led to believe (Vev could confirm this), that ULP will go 'off' at a slower rate if the container is full and sealed,kept out of sunlight and in a cool environment. Of course aircraft fuel tanks are NOT sealed, they have breathers and if the aircraft is not hangared, the tanks will not be cool. Possibly ULP in sealed containers will keep longer than in the aircraft fuel tank.

 

Vev can you comment please.