On the stale unleaded petrol (gasoline) myth

[RFguy]

This discussion excludes fuels with ethanol : 

****

I had a conversation with a 'petroleum scientist' at an oil company yesterday... yes ! To summarize :

1) When considering how old fuel is , volume/area exposure to air is the principal issue

2) Steel Jerry can sealed tight 90% full - good for 2 years with essentially indiscerable change

3) Plastic container- 90% full  : 6 months for the better thicker plastics . Plastic is not a perfect seal....

4) Vented container  - faster...

5) if 50% of tank volume is  air,  reduce the above time numbers by factor of 4

6) Octane increases slightly then flat lines - maybe 1 to 1.5 points- - Yes there is a octane boost perhaps 4 weeks of storage  at 90% fuel total volume . then no change. .Volatiles decrease, leaving octane boosting compounds in excess.

7) Vapour pressure (VP)  reduces slightly  at same rate (4 weeks at 90% tank Vol), then flat lines, however the VP variation is less than the summer/winter VP supply specification so dont worry about it.

8 )  Most premium fuels contain " detergency additives "  - " if oxidation processes are in place – non-fuel soluble compounds can form; this may also be more of an issue for carburetted engines – where the fuel jets allow continued evaporation etc. "

9) Loss of volatiles affects cold starting ability- significantly the  vapour pressure (depresses vapour pressure) . turns a winter gasoline into a summer gasoline.

10) Fuels may cause dissolution of tank seals, tank liners and so long durations in non -noble tanks  may cause 'unwanted compounds' to be added to the cocktail.

so- #8 is an issue for forming nasties in fuel jets, and potentially  items  like fuel  filters/gauze open to the air.

Edited November 23, 2022 by RFguy

[Methusala]

 I feel that I must add my long held and not very scientific view on this question. It is my belief that the IC engines that we rely on have been engineered such that age effects on fuel does not compromise their performance to an appreciable extent.

Most of us have encountered very old fuel and recognise quickly the foul smell and even the dark appearance. We recoil from such a mess. However, I have kept fuel in sealed containers for sometimes over 6 months before use in aviation. It has smelled, appeared and performed in a satisfactory way such that I don't worry greatly over its age. This applies to 2-stroke pre-mix, 95 pump auto petrol and avgas. In fact, I once did a job for a LAME and was paid in avgas that he was required to drain from GA aircraft and I  had no problems with it at all. 

I know that others are convinced to the contrary. Knowing this I have to say that this is merely my opinion and I am not wishing to tread on anyone's toes.

Edited November 23, 2022 by Methusala
correction

[skippydiesel]

RF Guy - you forgot to mention - adding a significant quantity (good dollop %?) of fresh fuel to the "old" in your aircraft, seems to restore almost all of the olds origional qualities.

[RFguy]

Hi SKippy.

No, I did not forget to mention that-

I deliberately did not mention it because according to this industry professional, that is not true.

 

Certainly addition of fresh fuel recovers a % of volatiles by volume, allowing generally easier starts. But it does not recover the aspects of the aged fuel such as oxidized detergents and additives that may leave solution, tank/vessel impurities now in the cocktail that are a bigger problem .

 

 

[skippydiesel]

There is reason/logic in you professionals opinion however I think you will find that the experience of a lot of ULP powered pilots is that a "refresh" works . The problem is that no one talks in % , so its all a tad speculative.

 

Time, as in how old is the fuel in the tank, also has a significant role in all this. I practised refreshing, by refueling before flight rather than topping up at the end. My guess -  I never had fuel in my aircraft that was more than two weeks old, before the refresh and the refresh would likely have been in the region of 50%.

[RFguy]

aircraft wing  fuel tanks are often large surface area voids ...probably worst case scenario. - tank breathers (which will breath diurnally) and huge surface area to volume ratio.  2 weeks is pretty good.

[RFguy]

Skippy, I guess the $64 question is, what happens in commercial underground fuel station tanks . Certainly there is effort (and it is indeed required) that the fuel doesnt sit there evaporating/ taking on air/ oxidizing. 

The answer is apparently - it depends. 

 

Good reason if using ULP to source from drums, ideally. 

[onetrack]

Keep away from drum fuel, it is the major source of fuel contamination. The only drum fuel to use, is fresh sealed drums, directly from an oil company depot.

I have plenty of experience with drummed fuel, I used thousands of drums of fuel in the 1960's to the 1980's. As soon as you open a drum, you open the door to contamination.

 

Underground fuel tanks in servos are at a constant ground temperature of around 15°, so no major problems as compared to above-ground tanks, where huge temperature and humidity variations exist daily - leading to contamination from moisture.

In addition, underground fuel tanks have highly restricted and controlled venting as they are not allowed to release large levels of volatile organic fumes. There are constant EPA checks on the level of volatile fumes from servos.

[RFguy]

Hi onetrack.

I didnt know 'drum fuel' was different from ' drum fuel' .  Open drum. use drum. I  assumed the only type of drum fuel is... comes in a drum from the oil company depot.  please explain.

[onetrack]

I'm talking about once a drum of fuel is opened, the potential for fuel contamination rises rapidly. Where it's stored, how it's stored, how it's sealed (drum bung seals can be pretty dodgy and bungs often don't get tightened properly).

You can store a part-used drum lying down on its side to reduce contamination from say a shower of rain, but the potential for contamination is still high, because drums heat up and cool down over  much wider temperature range than any other form of container - apart from a jerrycan in the sun. A wide temperature variation range experienced by a partly-full drum, is where the moisture levels can really rise rapidly, especially in humid conditions.

 

All I'm saying is bulk fuel is much less likely to contain contaminants, than a drum of fuel that has been opened.

 

Edited November 24, 2022 by onetrack

[kgwilson]

All of the things mentioned were included in the BP study of 2010 which I have posted on this site a couple of times. One thing your bloke did not say is that aged fuel (over 5 weeks old in a vehicle or appliance tank)  causes the mixture to be lean and the engine overheats resulting in detonation and piston damage in High Revving engines such as those found in chainsaws and boat engines. I can attest to this after one of the SES chainsaws would not start after being used once after sitting idle for 6 months. The report from the Stihl shop was "hole in piston, scored bore and big end damage" & the saw was trashed. This is not a major issue with a low compression low revving aircraft engine. I have started & run my J 3300A engine after well over 5 weeks without adding fuel & it started first turn of the key & ran well. This is not my normal procedure. That is to add fresh fuel every time I have 30 litres or less in the tank.

 

BP recommended a bit of fresh be used if the fuel has been in the tank for over a week. You do not need much but you will need more the longer the fuel has been in the tank. I keep our 8 SES emergency generators with very little fuel in them and always put 3-500 ml in the tank before going through the monthly generator test process. They are then run under load for 10-20 minutes.

 

The comments re plastic containers is very valid. I have a 4 litre ex oil container for lawn mower petrol. The light components permeate the plastic and the container stinks of fuel. The same is true for the vinyl ester tank in my aircraft fuselage. It dissipates quickly when I open the canopy after it has been in the hangar for a week. The heavy wall plastic containers are much better.

 

The pdf of the BP study is attached.

petrol-life-vehicle-tanks.pdf

Edited November 24, 2022 by kgwilson

[skippydiesel]

I try to source my ULP from servos that are likely to have a fast turnover in fuel, hoping this will result in purchasing  a "fresh" supply.

I always try for 98 RON, as I feel (that emotional bit again) that this may allow for some adulteration and deterioration, with the result being at least 95 RON - so far so good. 98 is also easier to find, as many servos no longer stock 95RON.

I store my fuel in Bunnings $20 plastic 20 L fuel containers and always try to use the full 20 L so as not to have small, rapidly aging /deteriorating, quantities left over.

I visually judge the sealing qualities of the container on their ability to retain the fuel gas ie does it bulge, about the same as its mates, during the warm day.

 

Onetracks sealed drums would probably be the best way to purchase known quality fuel  - the problem is that I would only be drawing off 20-60 litres at a time,  which would result in a less than optimum fuel quantity in the drum for an extended (deteriorating) period. I would also imagine the cost (inc drum & transport) may make the fuel cost uncompetitive with the servo "down the road"

 

Edited November 24, 2022 by skippydiesel

[RFguy]

Hi KG

I cant see the evidence that 'stale fuel' causes the mixture to be lean anywhere. 

according to this BP publication, the usage  actually riches up....

https://www.bp.com/content/dam/bp/country-sites/en_au/australia/home/products-services/fuels/opal-factsheet-storagehandling.pdf

see the air fuel ratio reducing (getting richer) 

Edited November 24, 2022 by RFguy

[kgwilson]

9 minutes ago, RFguy said:

Hi KG

I cant see the evidence that 'stale fuel' causes the mixture to be lean anywhere. 

according to this BP publication, the usage  actually riches up....

https://www.bp.com/content/dam/bp/country-sites/en_au/australia/home/products-services/fuels/opal-factsheet-storagehandling.pdf

see the air fuel ratio reducing (getting richer) 

Read the pdf of the BP report " Loss of light components - impact on mixture" section.

[RFguy]

Hi KG.  I think you've interpreted that document incorrectly. mostly it seems to be orientated to engines cold starting and high revving....

 

They are talking about issues with vaporization at cold temperatures causing lean mixtures at cold startup (like a chainsaw etc revving to life at high revs and suffering pre-ig and detonation) 

 

it talks about the mixture going RICH :

"Because the fuel carburetors and injectors operate on a volume metering system the higher density means that more fuel is introduced for a given volume of air and so the air fuel ratio is fuel rich. "

 

This does not cause LEAN mixtures in moderately running warm engines.

 

So, stale fuel does NOT cause lean mixtures under normal vaporization (engine warm) conditions.  The reduced vapour pressure (stale fuel losing its volatiles)  might cause difficulties in getting the correct mixture if the carb is unable to vaporize the mixture tdue to a very cold carby body.   In fact the vapour pressure of AVGAS is about~ 40 to 50 kPa versus 60-80 for ULP...  and we dont have vapourization problems with AVGAS.

 

 

Edited November 24, 2022 by RFguy

[Thruster88]

Because I live on a farm, for the last 39 years I have used 205l drums to refuel the thruster. Vented drum pump so not sealed. Could be up to 3 months old. Zero issues with any rotax 2 stroke. Never had a problem starting a honda single cylinder industrial engine even if they had been sitting for 6 months with out adding fuel.

 

The SES chain saw most likely holded the piston due lean mixture caused by carburetor fault, hard diaphragm in the pump or metering section etc.

[kgwilson]

26 minutes ago, RFguy said:

 

 

They are talking about issues with vaporization at cold temperatures causing lean mixtures at cold startup (like a chainsaw etc revving to life at high revs and suffering pre-ig and detonation) 

 

it talks about the mixture going RICH :

"Because the fuel carburetors and injectors operate on a volume metering system the higher density means that more fuel is introduced for a given volume of air and so the air fuel ratio is fuel rich. "

 

This does not cause LEAN mixtures in moderately running warm engines.

 

So, stale fuel does NOT cause lean mixtures under normal vaporization (engine warm) conditions.  The reduced vapour pressure (stale fuel losing its volatiles)  might cause difficulties in getting the correct mixture if the carb is unable to vaporize the mixture tdue to a very cold carby body.   In fact the vapour pressure of AVGAS is about~ 40 to 50 kPa versus 60-80 for ULP...  and we dont have vapourization problems with AVGAS.

 

 

Correct but it is when the engine is cold that the damage occurs. This is also when the most wear occurs in all engines. Engines with large tolerances won't be affected as much but modern engines have far smaller clearance tolerances as enabled by technology and better lubricants. Old sloppy 2 strokes will not be too bothered about lean fuel at cold start. They will just take longer to develop full power so they have time to warm up. With a chainsaw, most users start it with choke, once running, choke off, rev it to full power and start cutting. The bloke at the Stihl shop said they get a spate of failures usually in Autumn when people decide to get in Winter firewood and the chainsaw hasn't been used for 6 months to a year.

[Carbon Canary]

Any chance that SES chainsaw was filled up with petrol without 2 stroke oil added ?? 

I've nearly done it myself - a fuel tin for 4 stroke lawnmower and trimmer, separate pre-mix fuel tin for 2 stroke chainsaw and leaf blower.  Don't grab the wrong one !

[Thruster88]

27 minutes ago, Carbon Canary said:

Any chance that SES chainsaw was filled up with petrol without 2 stroke oil added ?? 

I've nearly done it myself - a fuel tin for 4 stroke lawnmower and trimmer, separate pre-mix fuel tin for 2 stroke chainsaw and leaf blower.  Don't grab the wrong one !

It would seize but unlikely to hole the piston. The piston crown is weakened due to excessive temps with a lean mixture, a hole soon follows.  

[kgwilson]

35 minutes ago, Carbon Canary said:

Any chance that SES chainsaw was filled up with petrol without 2 stroke oil added ?? 

I've nearly done it myself - a fuel tin for 4 stroke lawnmower and trimmer, separate pre-mix fuel tin for 2 stroke chainsaw and leaf blower.  Don't grab the wrong one !

No. All our fuel is managed & in specific "chainsaw" containers with the date of mix & ratio marked on the top & initialled by the person mixing. They are only refuelled with 50:1, 2 stroke mix. The problem was, it was not refuelled before it was used. I have modified our SOPs so a saw is always topped up with fresh before use. If it has not been used for a long time we tip the fuel in to a glass jar. Fresh 2 stroke is green (from the 2 stroke oil used) and clear. Old 2 stroke is brown and can also be cloudy. Earlier 2 strokes were 20 or 25:1 ratio so overheating was more difficult. You always knew the fuel was 2 stroke as the exhaust showed some smoke even when hot. The Stihls only show a minimal amount of smoke right after start up.

[Yenn]

I have twice tried to use mogas in my Jab engined Corby. It goes well when first filled but soon becomes difficult to start and if persisted with will cause detonation. Seems to be less than a month for deterioration to become apparent.

[RFguy]

Yenn : "will cause detonation."

does? or will ?

[kgwilson]

Even 91 octane is unlikely to cause detonation in an 8:1 compression ratio engine. Something else is awry. I have only used Avgas when away when I could not get Mogas. 95 or 98 runs beautifully in my Jab 3300 engine & the engine stays clean. I don't top up oil between 25 hour changes and the oil is still relatively clean after 25 hours.  I have seen numerous Avgas only Jab heads & pistons with so much hardened gunk on them I am surprised the engine still runs & the main reasons the head was removed was because of a valve, compression or ignition problem.

[Mike Gearon]

8 hours ago, kgwilson said:

Even 91 octane is unlikely to cause detonation in an 8:1 compression ratio engine. Something else is awry. I have only used Avgas when away when I could not get Mogas. 95 or 98 runs beautifully in my Jab 3300 engine & the engine stays clean. I don't top up oil between 25 hour changes and the oil is still relatively clean after 25 hours.  I have seen numerous Avgas only Jab heads & pistons with so much hardened gunk on them I am surprised the engine still runs & the main reasons the head was removed was because of a valve, compression or ignition problem.

This again raises the avgas versus mogas question. I’ve recently been told about the Soar Aviation aircraft at Moorabin running exclusively Avgas with no problems around the head. 600 hour opening the gearbox and cleaning out the gunk.

 

While I believe this and it makes me feel better about occasional avgas fills on the rotation engines I have now and the new UL engine coming I see the above post that confirms what I’m reading from USA Rotax forums…….Don’t run avgas in the lower cylinder head temp engines.

 

On the subject of temperature. It must be the higher cylinder head temps on Engines that don’t have a head gasket. Rotax/ UL run lower temps. The Lycoming/ Continental engines must be burning the lead at that higher temperature. 
 

Conclusion. As I might have mentioned elsewhere At OSH Kosh I talked with the rotax guys. One told me he puts avgas in everything he owns when it’s going to sit for a while. That’s 2 and 4 stroke small engines. Volatiles as discussed in this thread must be more stable in avgas. 
 

The ULPower engineers at Osh Kosh told me their steel cylinder engines handle longer periods without rust when run on avgas prior to prolonged shutdown.

 

I’d like to see the things first hand. I’m bloody confused…Anybody have pics of rotax cylinder heads run for many hundreds/ thousands of hours exclusively on avgas? I’m becoming more convinced it’s better to run Avgas and pull the gearbox of a rotax down at 600 hours because you want to check the slipper clutch anyway. 
 

At the moment I’ll more happily fill avgas on cross country and longer shutdowns and try for fresh mogas. The Pipistrel motor glider has fibreglass tanks that melt if part ethanol fuel is used. They changed this for newer Pipistrel’s. I’ll definitely run avgas in the 2003 motorglider.

[RFguy]

agreed with you KG and Mike  .  fresh ULP is the best fuel for a Jabiru. Jabirus have a top end TBO of about 300-400 hours on avgas (without additive)  .  Anecdotally, adding Decalin Runup at about double the minimum dose reduces avgas buildup 90%....  Another anecdotal- I hear about more stuck rings and buildup issues in engines that are 'babied', and more so the 6 cyl jab, not the 4 cyl, presumably the 6  has plenty of power in hand for most users and users might back off below the 26 lph. 

 I saw a 2200jab (800hours TSO) that was dieseling on shutdown last week. something glowing in the chamber (some of that gunk I guess), and the ULP was old- 75% of it in the 60 litre tank  was 6 to 12 months old.  the first fuel drop looked like a banana milkshake (same colour as the fibreglass inboard tank)  as it hadnt moved in a while. subsequent drops got cleaner until it looked like ULP..... Cold compression was OK,   (estimate in the high  60s) , But, it started quickly - kicked first revolution- no issue at all  and did runups just fine. cant remember static RPM  . suggested to the owner to drain it all, put a fresh tank of avgas is it and see what it does. (still, at 800 hours TSO it needs heads cleaned and rings /ring lands done) 

 

Mike, when I get the big jab flying again long XC likely at fill home  with ULP95/98 (whatever has the lower RVP)  then  at destination, go to the bowser and fill to the tabs with avgas before parking.