A General Aviation News reader has a problem: “My fuel supplier accidentally added 100 gallons of 87 octane unleaded, no-ethanol gasoline to 300 gallons of 100LL in my storage tank.”
“I’ve read that the lead in leaded fuel raises the octane level of unleaded fuel,” he continued. “But how much? What octane do I now have? Do you have any idea?”
It’s a very interesting question, but unfortunately I cannot give him an accurate answer.
At the outset there’s two problems. The first is that the octane response to lead will vary for different fuels. The second: We do not know the original octane of either fuel.
But let’s try to break this down to find some approximate answer.
First, we can only consider the lean or motor rating, because the rich rating of the car gas is anyone’s guess.
The lean rating of the 100LL would be about 100, although many samples I have tested were above that because the refinery had to add additional toluene to reach the 130 rich rating requirement. That additional toluene raises the lean rating above 100.
The lean or motor rating for the car gas would be only a guess. Most 87 R+M/2 have a sensitivity of about eight. (The sensitivity of a fuel is the difference between the Research and the Motor ratings.) That leads us to assume the Motor rating would be about four numbers below the average, or 83.
If we could just average the octane of the mixture in his storage tank, we would have three times 100, plus 83, divided by four, which takes us to 95.75.
If we consider that the lead amount would be mixed completely, then all of the fuel would have about 1.5 grams of lead per gallon.
But the octane number of a fuel because of the addition of lead is not a linear curve. The first half a gram of lead increases the octane more than the next half a gram or the next and so on.
A rough estimate would be that the two grams of lead in the 100LL would have increased the octane about six, or maybe seven, numbers at the refinery. If we go with six, the first half a gram would increase the octane over two octane numbers, while the fourth half a gram would only increase the octane by about one number.
If I assume that both fuels have the same lead response, then the original 100LL would be about 99 and the car gas with 1.5 grams of lead per gallon would increase about five numbers to 88. If we average the octanes out, we would take three times 99 plus 88, which equals 385. When we divide this by four, we get 96.25.
So, we gained about a half an octane number for the mixture over just a straight average. This does change the octane number of the mixture some, but not enough to get it to meet the 100LL spec. And remember, there were a lot of assumptions needed to get that number.
Here we get down to his real question: What can he do with the fuel mixture in his storage tank?
He cannot burn it off in his automobile unless the vehicle was manufactured before 1975.
In aviation it will come close to meeting the 91 octane fuel spec. It also comes close to meeting the 80/87 spec, except for the lead and maybe the vapor spec.
Technically it should work well in an 80/87 or 91 octane requirement aircraft.
However, if he wants to sell it, it may not meet all of the legal requirements.
Too big a liability to sell to aviation, no road taxes so cannot sell to ground vehicles. Make the distributor take it all back clean your tank and resupply.
If there is alcohol in the fuel it should not be used in planes with the auto/fuel STC according to my mechanic. The auto/fuel STC predated the addition of alcohol in auto fuel.
Out of curiosity, and not wishing anyone a bloody nose, did anyone ask the FAA? I’m guessing that contaminated fuel is scrap POL. Possibly with a tank wash/rinse?
Any aircraft with an Auto Fuel STC can burn the mixture. The STC covers mixing the fuels. I’ll give him $3.00/ gallon.
This part caught my eye:
“It also comes close to meeting the 80/87 spec, except for the lead and maybe the vapor spec.”
Any chance this would have a “worse” RVP (i.e., more likely to vapor lock) than regular 80/87? My guess is “no”, but I’m curious what you think.
From a lead standpoint, I would guess it would be no worse than running an 80/87 motor on straight 100LL
I think this quote is the best possible outcome:
“Technically it should work well in an 80/87 or 91 octane requirement aircraft.”
Discount the crap out of it so it’s not a total loss. Maybe include a warning: If you buy this to “stock up” because of the price, don’t let it sit around for 6 months! Use it!
To bad this did not happen in Spring when having a blend with some 100LL would help overcome any “winter blend MoGas on a hot day” issues.
A couple of years ago, an auto-fuel blender/distributor in the US Midwest inadvertently introduced too-much alcohol to raw gasoline… normally 10%+/-2%… some fuel was contaminated to well-over 20%. This fuel was distributed over several days to mom/pop/cheap gas stations all-over the area. The notifications/recall/system-purging and auto repairs made for a VERY costly financial and ‘good-will’ [TRUST] penalty… and was a regulatory-quagmire… for the fuel blender/distributor… and was ‘front-page-news’ for weeks. Thank God I didn’t get gas from any of these stations.
Odd comparison from From the National Lubricating Grease Institute [NLGI]. Testing revealed that when two hydrocarbon greases of different specification were mixed together the relevant properties of the two merged to become the ‘worst’ properties of the two greases above 80-20 mix ratio. Most importantly affected are ‘pressure rating’ and operating temperature range’; to a lesser degree corrosion protection due to moisture absorption.
Hypothetical example.
Grease A = 10000-PSI pressure rating, -100F to 250F operating temperature range.
Grease A = 20000-PSI pressure rating, -40F to 350F operating temperature range.
Grease A+B = 10000-PSI pressure rating, -40F to 250F operating temperature range.
Come on Ben, don’t leave us hanging! What did he do with the gasoline? Frankly, it’s not his problem to solve, his supplier has the liability for this one. He is darn lucky that fuel never got in some folks aircraft.