In my last column, I proposed a way to get the lead out of 100LL — or at least out of the name.
I received a number of responses, both pro and con. I always appreciate both types of comments and try to learn from them.
I would like to address just one of the negative comments. One reader noted that my concern for the old radial engines was misplaced because these engines were not qualified on 100LL.
There were one or two applications of World War II aircraft that were certified on 115/145, but almost all of them were certified on 100/130 leaded avgas.
And here is where a lot of confusion comes in: 100LL is actually 100/130 low lead avgas as defined by ASTM D-910. Yes, 100LL does meet the 130 rich rating and all other properties of 100/130, except for a limit of 2.0 grams per gallon TEL and a different dye color. So almost all radial engines ARE qualified on 100LL.
The difference between 100LL and 100/130LL sounds insignificant, so why is it that important to note?
First, one must understand that an octane rating of a fuel is not a measurement of a physical property, but rather a performance rating of the fuel under certain prescribed conditions.
For example, a motor octane rating of 100 means that under standard operating conditions at 900 rpm, the test fuel had the same knock intensity as a 100% isooctane fuel in a CFR test engine.
Saying all 100 motor octane fuels are the same is like saying all 225-15 tires are the same. This does not take into account tread width, tread design, rubber compound, radial or bias construction, and on and on.
Way back in history, industry technical experts got together and formed a committee to develop a system to rate the anti-knock characteristics of fuels. The test method they developed was called the research octane test. It quickly became the industry standard.
But as the committee started to rate fuels in actual cars, the engineers found that there was a poor correlation between the research test number and how it performed in real cars. That led them to change the conditions of the test by raising the speed of the test from 600 to 900 rpm, as well as the intake air temperature. The new test was called the motor octane test. The results from this test correlated to real world cars significantly better than the research octane test. But the research number was higher, so the industry decided to keep both.
In aviation, another committee developed the lean rating. It was very close to the motor method, so they just used that test with a simple conversion table. However, as power increased — especially from supercharging — the lean rating became a poor predictor of anti-knock performance in actual service.
The committee then developed the rich or supercharged knock rating test, which correlated much better with real world operation. And so was born the two octane number system, which has served the aviation world well.
Now, people are telling us that they can replace 100LL with an unleaded fuel as long as it has a minimum of a 100 lean rating octane. There are a few problems here.
The first is lead bonus. Leaded fuels provide several octane numbers higher anti-knock performance in the real world than an unleaded fuel of the same rating.
The second is that the rich rating does not work with unleaded fuels, so that different fuels with the same lean rating may provide very different performance, especially in the big radial engines.
Third, the present system works because all companies produce 100LL from basically the same components.
And fourth, when they start making unleaded fuels with different compositions, a lot of questions will be raised, such as how will they blend and what will the properties of the mixture be?
There are also a lot of other minor problems, which I will address later.
But I firmly believe that developing a new unleaded fuel based only on motor octane number is like trying to pick a new wife sight unseen and based only on her weight. It may work out, but I would rather stay with my current wife, who isn’t perfect, but she puts up with me.
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When I operated a part of a refinery, I was a Process Operater. We were able to make 115 octane no lead gasoline in the Platformer. This is expensive as the catalyst must be regenerated much more often. This we regularily produced around 100 octane no lead and used it to blend and upgrade low octane fuels.
A very succinct, accurate and informed explanation Ben.
I am very confused by your article. To me you are mixing apple and oranges. The lead content does not increase or decrease octane and octane level does dot increase or decrease the lead content. Your article seems to infer that it does. Please explain.
” which correlated much better with real world operation ”
Ben,
What real world testing is actually occurring now ? As an Engineer, I know what looks good on paper is the starting point of ” Development ” . How reliable is this phase ? What engine types and configurations are being tested. Where ? In a test cell or will there be people flying them first before I do ?
Good topic and discussion. My old 172 is gone, but I still try to keep up. This is a little like the ADS-B discussions. Reality is upon us and a preferred road to the future needs an industry/owner wide solution soon. I hope more knowledgable folks add to the substantive input soon.
This is the best ever, love the analogy of the wife.
Ben, this one one of your best and most informative columns ever. I’ve spent some time researching the research and motor methods of octane rating, but never found a good explanation of the rich and lean dual number ratings. It’s important to understand how we got to where we are so we can avoid the mistakes of the past as we move forward. Thanks for the history lesson.
Unleaded fuels are where most of us need to be, but as pointed out, this might be difficult for some of the warbirds with turbo and supercharging as well as a few other high performance planes. If one can run unleaded, there are a lot of bonuses with reduced engine wear and reduced maintenance costs. This is where many of us are likely to end up paying the price for an expensive boutique high octane fuel so we can continue to support the few high performance engines when all most of us need is an equivalent to high octane premium car gas with better storage qualities. Unfortunately, the EPA and liability lawyers have made it prohibitively expensive for low volume FBOs to carry more than one type of fuel for piston aircraft. There is no incentive for the FBOs to carry dual fuels as the less expensive Mogas sales are mostly at the expense of the more expensive 100LL sales. So we continue to wrangle over the regulations to force feed all of us a single drop in replacement in order to avoid stocking dual fuels. For the foreseeable future, I see myself continuing to haul my own premium unleaded rather than buying 100LL, or the likely to be more expensive “drop in” replacement.
Good analogy on the wife. Mine not only puts up with me she can cook too!
As for the fuel turbo charged and radial engines will maybe require additives
mixed with the new fuels.