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.