In the 1970s, the automotive world switched from leaded to unleaded fuels and the oil companies did a lot of research on knocking and how to prevent it. One of the big projects involved octane requirement increase (ORI).
In this program, cars were rated for octane requirement when new and then every 2,000 miles. The octane requirement increased until it leveled off at about 20,000 miles.
Usually the ORI was about 6 to 10 numbers. We then removed the heads and cleaned the deposits off all of the pistons and re-checked the octane requirement. The requirement usually dropped a number or two.
We then removed the heads and cleaned the heads and valve faces. When we re-checked the octane requirement, it had dropped another number or two, but still was not back to new levels.
The really interesting part came when we removed the head a third time, took the intake valves out, cleaned the deposits off the under head part of the valve, but not the seat. When the engines were reassembled, the octane requirement of almost every one dropped a number or two.
This means that the change in intake flow pattern caused by the intake valve tulip deposits were enough to increase the octane requirement of the engine by 1 or 2 numbers.
The ORI for passenger car engines is significantly higher than for aircraft engines because of ash-containing oils and non-bottomed fuels. But the ORI for aircraft engines can still be significant.
A second interesting point was that when we would purchase a pair of identical cars, their octane requirement was usually a number or two different. So here we have two identical engines produced on a mass production line, but when they got into the field, their octane requirement was different.
In aviation, we have a lot of different people assembling engines, and we add an additional variable with PMA parts. The FAA is testing a particular model of aircraft engine with the new unleaded fuels and expecting that this test will cover all of the combinations of original and PMA parts that will be seen in the field. Will a cylinder head with a different casting method and port design produce the same octane requirement as the original manufacturer’s parts?
To compound these problems, pilots need to adjust the mixture strength during flight. In an operating aircraft engine, the carburetor (or fuel injection) does not supply a completely homogenous mixture to all cylinders.
Most pilots know the proper way to lean out their mixture strength, but many others do not even know what a carburetor is, let alone how to lean it properly.
At the stoichiometric air/fuel ratio, the temperature and pressure is at the maximum and continued operation here will lead to knocking and possible engine damage.
In the real world, pilots will lean until the engine runs rough and then richen up until it smooths out.
With the problems described above, which can change the A/F ratio distribution in a given engine, plus improper leaning procedures and a very small octane safety margin with unleaded fuels, this can lead to one or more cylinders operating at a dangerously high temperature.
Another problem is that a lot of planes are equipped with single point exhaust gas temperature gauges. This assumes that one cylinder is always the hottest and as long as the pilot leans to keep this particular cylinder cool enough to live, the rest of the cylinders will be OK.
But if deposits or different parts change the cylinder to cylinder distribution, will the “chosen” cylinder still be the hottest? Will it be necessary for all PMA parts to be tested for their effect on octane requirement of the host engine?
In the future, if an engine has a knocking-related failure on an unleaded avgas, who will the lawyers sue? Will it be the fuel supplier, the FBO, the engine builder, the PMA parts supplier, or the FAA?
It really is a trick question because we know they will sue all of them. The devil is always in the details.
Ben:
I had the privilege to work with you at the Shell Oil Research Lab, in Woodriver, IL. We worked on you Buick Grand Sport together, and on a cold and rainy fall day we ran the old Shell Mileage Marathon on the Great River Road just north of Alton. Wow, what great memories, and what a wonderful experience to add to my things to talk about of my wonderful life in this industry that we call “Automotive”.
Thank for those memories Ben, and for the right to tell people that ” you are a friend of mine”. I would love to hear from you.
Sincerely
Bill Launius
For those of you who think “there should be mo-gas available and FBO’s have no incentive to provide it” I have a suggestion. Go to your local airport authority, whomever that might be, and suggest that you’re willing to install a fuel system, at your cost, to provide mogas. All you need is permission to do it and all you ask is that you be allowed to make a little bitty profit on the sale of the gas. I suspect the local FBO won’t oppose your request as long as you’re required to meet all the same state and federal requirements in the installation of your fuel farm that he’d have to meet. Then you can go out and start putting the numbers together. When you find that the fuel farm will cost you, at a minimum, $10,000.00 in equipment cost alone, then you can figure that the first load of fuel will cost you over $4.00/gallon, at today’s prices, and you’ll have to buy a minimum of 1000 gallons (most likely you’ll have to contract to buy a minimum number of gallons/per year and your cost/gallon will be based on that minimum) and you can start to figure out how much you have to sell the gas for. You need to pay off your investment, interest and principal, buy the gas and make a little profit which you have to keep on hand for the day the price of gas goes up from $4.00 to $5.00 a gallon, or more, during the 3 months it takes you to sell your 1000 gallons to people who are buying, on average, less that 25 gallons at a time. Get the picture? There’s a reason FBO’s don’t have any incentive to install 3 grades of fuel, but you’re welcome to put in your mogas farm if you want. I bet you won’t.
I operate a good old Continental C-90 in a PA-11 built in 1946. A simpler engine would be hard to imagine. I did the STC thing for unleaded fuel/auto fuel years ago. Despite the changes and reassurances that all will be well, I worry about the lack of any lead in my fuel. I worry a lot more about getting ethanol in the car gas so i test each batch for this and so far as I know I have never had any ethanol in my fuel.
I find that if I run a mix of about 20% LL-100 Avgas with regular Petro Canada car gas things sound and work well. My plugs last, the engine runs well and as far as I and my mechanic can tell all is well. I am, however, a low hour per year recreational flyer so maybe my method is not for all to use. For example I fuel from 5 gallon cans so mixing fuel types is relatively easy for me.
I avoid ethanol mixed fuels for ALL my antique machinery, from tractors on up to my plane. I don’t think it is a good idea, period and the only place where I don’t care about ethanol is my relatively new car, which I am told, Ford built to take this concoction.
For what this is worth to people.
Ben, Why are you so negative? Why do you think that FAA will not test for all the combinations you mention. We can land on the moon, asteroids and soon Mars but we can’ get the lead out. Why are you using scare tactics? Do you have a financial interest in keeping things as they are? The whole world will get rid of leaded gas but many Americans will not change.
after the do gooders do some more good, is there ANYONE who can tell me what the measureable difference will be to the air … ANYWHERE ( lead levels due to piston aircraft ? hahahaha )
and if you want to attempt to make a do gooder THINK, ask them, ” after all the carbon you want to prevent me and the WESTERN WORLD from emitting, what will be the MEASURABLE DIFFERENCE ? ”
here’s a hint, ( the dates are not precisely correct, but the concept is DEVASTATING ) whatever global warming that would have occurred by January 2050, will instead occur in June 2050.
in the mean time you will have sacrificed your way of life while the rest of the world, ie, the bric country’s, will be laughing at you the entire time, figuring the way you got so stupid is because you listened to your professors who told you how smart you were for regurgitating what they told you. fwiw, i don’t have any issues with regurgitation, it’s how birds feed their nestlings.
and no i’m not an angry ( young / old ) man … i get to go flying my own airplane tomorrow for over 4 hours. and i will enjoy, savor, every single MOMENT.
My favorite quote these days ” We have now sunk to a depth at which restatement of the obvious is the first duty of intelligent men “. George Orwell
PS i need leaded fuel, my cylinders have 10 to 1 compression.
Good and sensible comment by Greg W.
The ongoing work and discussion on unleaded avgas shows the importance of communicating with the FAA – however difficult, even futile, that might seem to some – via individual and GA organizational action and responding to FAA requests for comments and NPRMs.
Regarding those go-to-villain lawyers – of which I am one, but not a villain – strictly speaking they would not sue anyone: they would represent someone (a “plaintiff”) who incurred an injury or loss from a knocking-related failure and was suing those whom he or she had reason to believe caused or contributed to that failure, hence to the injury or loss. The goals would be to compensate for that loss and provide incentive to improve the product or process, to reduce the risk of harm to others. And, the fuel supplier, etc., would have lawyers representing them to protect their rights and interests. (For the record, in private practice I was almost always on the defense side; in airport administration, I have worked closely with GA and bizav pilots and entities and have enjoyed them thoroughly. We legal types are not all bad, really!)
The details are indeed the problem and those problems could be simplified by simply having two grades of avgas. There is little reason that +80% of the aircraft that can use less than 100 octane are held hostage by those that do need it. If an auto station can have three grade of gasoline and e85 and diesel an airport could have three fuels, low oct., high octane and jet A. The FAA has approved 94 UL,(100LL without lead), it could be used by the 80% right now with out additional approvals. I am forced to use 100LL that was not approved in my aircraft with all of the lead fouling problems because it is what a small vocal group say “we” need. I need 80/87, max .5 gm/gal lead most was much less than that much of it unleaded. My other aircraft was certified with 73 Army avgas, an UNLEADED avgas.
Politics are the problem the real technical problems would be rather simple to overcome.
Greg W hit the nail on the head! I totally agree. I have burned 87 and 91 octane Mogas for years in Continentals and Lycomings and buy it anywhere that sells it on x/c trips. I have full engine monitoring and see no difference in performance or temps between 100LL and Mogas. My plugs burn much cleaner with Mogas. This is a no brainer being over complicated by technical scare tactics, self interests and politics. Mogas is available now and has had STC’s available for years that went through exhaustive testing. Solve the high compression engine problem and don’t penalize the majority of planes that run fine on what is already available at an affordable price. It just needs to be made more widely available and there seems to be little incentive for FBO’s to offer Mogas in the current environment.