Octane Degradation

[Neil_S]

Hi guys,

 

I think this may have been touched on before in some other threads, but I have a specific question that perhaps someone can answer. At what rate does unleaded petrol's octane rating degrade? Is it just based on time, or do other factors come into play (eg temperature)?

 

I usually buy 98 octane PULP for our plane, but sometimes it sits in my garage for weeks before I refuel with it. Basically I would just like to have a comfort factor wrt how long I could theoretically store it before it became unsuitable for use in our Rotax 912.

 

Thanks in advance,

 

Neil

 

 

[Vev]

Hi Neil,

 

Temperature and the quality of the seal on the container will have an effect on how long you can store fuel. If you store your fuel in a well sealed container and keep it in a cool place, which shouldn't be hard at Hoppers Crossing this time of year, it will be fine for a 2/3 months without issue... 1 month less for summer.

 

Fuel can degrade over time owing to two main factors (there are others but less of an issue), firstly the light fractions in the fuel evaporate if exposed to the atmosphere where you will end up being left with the heavier components and lower octane.... Secondly is oxidation, often stated by heat stress, where the fuel basically breaks down and runs away into auto decomposition. If you keep your fuel in a cool place in a sealed container you get around 95% of the issues that destroys octane values.

 

However at the end of the day, "it is always best to use fresh".

 

Cheers

 

Vev

 

 

[Guest davidh10]

Vev would be the person to definitively answer, but I think you are pretty right if you keep it in sealed jerry cans, as the loss of octane for ULP is chiefly via evaporation, however oxidation is also a factor and thus so is heat.

 

See the discussion in this thread. There was no definitive time given.

 

Mine gets stored for up to a month and a half in sealed jerry cans, and I use ULP (91 RON) for my 912. No issues. In good weather, the three jerry cans get turned over in a couple of weeks, but if we get some prolonged bad weather or I have other commitments, then it sits around a bit longer.

 

Edit: There you go.. Vev gave you a better answer while I was still typing :-)

 

 

[Russ]

OK.............are those "octane booster" adatives.....................good, or money wasters

 

 

[Vev]

OK.............are those "octane booster" additives.....................good, or money wasters

Hi Russ,

 

Good question.... I'm not a fan of adding anything to fuels to boost octane as there are too many uncontrolled and unknown elements to manage.

 

Not sure why you want to bump octane? Although there is an alternative, you can blend Avgas into Mogas to boost Mogas octane, there is no unknown compatibility issues with this approach. The octane bump is greater than the normal linear blend ratios as the lead in the avgas has a synergistic effect on the octane in mogas. Additionally, avgas will also reduce the volatility and slow down evaporation and improve vapour lock issues. However there is a down side, you will get lead some deposition in the combustion chamber and you may have an increased oil change frequencies.

 

Cheers

 

Vev

 

 

[Guest davidh10]

...you can blend Avgas into Mogas to boost Mogas octane...However there is a down side, you will get lead some deposition in the combustion chamber and you may have an increased oil change frequencies.

As an example, Rotax SB says that if you use Avgas 30% of the time, then you need to adopt their Avgas maintenance intervals, which includes halving the oil / filter change intervals, but also affects spark plug replacement and overhaul intervals too.

 

 

[facthunter]

Not mentioned is the mixing of fuel with oil. ( two strokes) Once mixed, don't keep it very long before using it. With mogas always buy from a branded outlet that sells a large quantity of fuel. ( and has a reputation for not putting other things in it. Nev

 

 

[Neil_S]

Thanks guys, especially Vev, appreciate your inputs.

 

I was basically going to keep the fuel for up to a couple of months, then put it into my car if not used, so you have pretty much confirmed that would be a reasonable approach.

 

I use the 10 & 20 litre containers sold at Auto stores specifically for petrol storage (Australian Standard nnnnn), so I assume they are okay.

 

Cheers! :big_grin:

 

Neil

 

 

[Sloper]

l have used Stabil in fuel for 2 of my cars that may not get used for a couple of months, with great results.

 

One a low performance moter and the other hi performance.

 

Its the one in the avatar that is the worst.

 

l also beleive it is recomended for use in aircraft engined by the SAA in the States.

 

regards Bruce

 

 

[Guernsey]

I also have problems sometimes when I have not flown for some time and worry about 'stale fuel' . I use normal unleaded two stroke so on one occasion I had fuel in plastic 20 ltr fuel containers which were stored for 6-7 months. I had a Supa Pup with a fuel tank in each wing so I put fresh fuel in one and the stale fuel in the other then went flying. I did straight and level plus climbing at a steady rate and kept switching from one tank to the next with no discernable difference in performance. I am not advocating that you do this but just telling you all as a matter of interest.

 

Alan Marriette.

 

 

[Vev]

Hi Alan,

 

It is highly possible your fuel may not have gone stale over that time frame if all of the conditions were ideal. However there are factors that could change things quickly and you could end up with a very different outcome. Long before you end up with a sticky goo, you will get a degradation of octane which will cause detonation ... this detonation will cause a huge amount of damage that is irreversible and could lead to catastrophic engine failure.

 

Personally I would rather play things safe and use fresh fuel and only store for a short period and blend your 2 stroke oil the day you use it ... why risk damage to your pride and joy as well as your own and pax wellbeing.

 

There's a very old saying ... there's no fuel like new fuel and old fuel is like no fuel.

 

Cheers

 

Vev

 

 

[airangel]

I am absolutely convinced that fuel will " go off" far quicker in "plastic" jerry cans, that in a proper metal container. Put a plastic can in the boot of the car for a few days and notice the fuel odour in the the vehicle. not so with metal container. A component part of the fuel exudes thru the plastic. [in my humble opinion]. I am refering to "approved" plastic containers.

 

 

[facthunter]

Guernsey, a trial like that would not be conclusive. Premixed two stroke fuel is a trap. Whether it does destructive damage to an engine on a particular day does not prove much. There are other factors as well as detonation. Ring gumming. as an example.

 

With detonation you can be right up to where it is just about to happen and once sometthing tips the balance,( a bit of incandescent carbon for instance), you have detonation and more heat because of it and away you go. Caved in piston crown or rings burnt into the side of the piston, and you're looking for somewhere to put it down quick. Nev

 

 

[Guernsey]

Thanks guys for your valuable imput fortunately I only tried it once as a trial to see if there was any loss in performance. I normally put my old fuel in my two stroke generators and drain the tanks on the aircraft. I hope this thread will help a lot of us which is what makes this forum so VALUABLE. Well worth the $50 a year to keep this site going. Thanks Ian.

 

Alan Marriette.

 

 

[facthunter]

Alan I must add that I don't intend this as a personal thing either. ( and you didn't take it that way either , thank you) Just an opportunity to get the message across. Over many years of MX tuning and involvement I have seen quite a few good brands of engine come to grief. In most cases I believe that the cause was stale fuel. IN flying there is more sustained load on the engine although it is not in such a high state of tune. A few failures I think are the result of having NO oil. Motors today tend to run with a pump, so no requirement to mix oil. Manufacturers have to make them as foolproof as possible. I still prefer to mix the oils myself. Running too hot a plug can be the cause too, but not talked of much these days. Also IF you add oil instead of using the pump, always enrichen the mixture to compensate for the viscosity change of the mixed fuel. Nev

 

 

[Russ]

thanks vev...........was hopeing you would jump in here...............2nd query...........aircraft spruce sell a fuel addative " Decalin", marketed to remove lead from avgas, tell tale sign of function is white residues out your exhaust. EFI engines with their 02 sensors etc, are not happy with to much lead in the system, emergency use is ok, but get back onto lead free asap...............so..............what's your take on this "decalin"

 

 

[Vev]

thanks vev...........was hopeing you would jump in here...............2nd query...........aircraft spruce sell a fuel addative " Decalin", marketed to remove lead from avgas, tell tale sign of function is white residues out your exhaust. EFI engines with their 02 sensors etc, are not happy with to much lead in the system, emergency use is ok, but get back onto lead free asap...............so..............what's your take on this "decalin"

Hi Russ,

 

Decalin does scavenge lead for the combustion chamber and works ok, but it doesn't remove lead from the fuel ... the white is usually a formation of lead phosphate from additisation of Decalin (decahydronaghthalene) as apposed to lead oxides from the normal lead build up we see in the combustion chambers. Be mindful you will still get lead oxide combustion contamination into your engine lube and will still impact on oil change frequencies in Rotax engine and other engines for that matter. (I'm sure Nev will give better guidance on engine service frequencies re combustion contaminates).

 

However before you add this to your fuel I would check with the oil company and ask them for their view and make sure there is no compatibility issues.

 

Cheers

 

Vev

 

 

[Russ]

sorry...............last query..............what oil blend stands out as the betterer .............all synthetic.....or those blended ones....................or it doesn;t matter if you do regular changes.

 

thanks.............russ

 

 

[Vev]

Hi Russ,

 

I presume you are asking about 4 stroke engine lubricants.

 

This is a very complicated subject and one that could take a lot to explain .... I will try and keep it short and sweet, so forgive me if I just skim the surface.

 

The different base stock of oils have pros and cons ... Group 1 mineral base oils are cost effective, have higher aromatic qualities and tend to absorb contaminates and clean through better solvency. Hydrocracks base oils have slightly lower solvency than basic mineral oils but are more stable and have a lower resistance to oxidation from thermal stress. Synthetics are more stable to oxidation but have a lower capacity to absorb and dissolve contaminates, they are quite expensive compared to ordinary mineral base oils.

 

Some lubricant manufactures try to find the sweet spot by blending mineral base oils and hydrocracks to get both the solvency and stability as well as a lower costs ... I think this isn't a bad approach overall. Some go all the way and use 100% PAO (Polyalphaolefins), which I think is more about marketing than a technical requirement, although I know some will argue differently.

 

One needs to think of the base oil as the carrier for the additive pack ... the additive pack is the real performance improver and has the biggest over all impact on how well an oil lubricates and protects an engine from wear and corrosion.

 

No matter what approved lubricant you use, you can't beat a higher frequency oil change .... by doing this you stay ahead of oxidisation, additive depletion and get rid of the abrasive and oxidisations causing contaminates.

 

Oil and filters are cheap ... I say change them often.

 

Cheers

 

Vev