The trick to blending fuels

After a review of the notes I received last year, the number 1 area of interest again was octane and engine knock.

The latest question was about blending octane number. For example, if you blend a 100/130 (commonly referred to as 100LL) fuel with a 90 octane mogas, will a 50/50 mix give you a 95 octane fuel? The answer is possibly, but probably not.

I know that this is kind of a trick question, but the answer can be useful.

The most important thing to remember is that the octane of a fuel is not a physical property of the fuel, but rather a performance property.

A physical property of a fuel or oil is like the viscosity of oil. If you mix grade 80 oil 50/50 with 120 grade oil, you will have grade 100 oil.

As a performance property, a given octane fuel can perform differently than another fuel when compared in different applications.

When comparing octane, you must consider that there are four different octane rating procedures.

In aviation there is the rich rating and the lean rating, but in mogas there is the research and motor rating procedures.

The lean rating and the motor method are close and there is a correlation table for them in the ASTM D-910 spec. This correlation works for avgas fuels made from alkylate. It may not work as well for other fuels.

The aviation rich method and the research method have only a rough non-linear sort of relationship.

Let’s do some calculations for our example. For mogas with a R+M/2 of 90, the motor rating will usually be four to five numbers lower or, say, 86. If we average that with the 100 from the 100LL, we would have a lean rating of, say, 93 for a 50/50 blend.

But blends like this can have a significant variation depending on the composition of the mogas.

Plus there is a lead bonus. Lead will increase the octane of a fuel in a non-linear way.

If the lead is mixed evenly, the addition of 1 gram/gal lead to the mogas should be more than the loss in octane from the reduced amount of lead in the 100LL.

But this will vary because of differing lead response for the mogas.

So what is the final answer? I can’t be sure, but it will probably be in that 93 plus or minus one to two numbers range. That is the lean rating.

What would the rich rating be? That is really a guessing game.

The rich rating of many unleaded fuels can be misleading, so the finished rich rating may not correlate to the real world.

The really big question is how the mixture will perform in a real world aircraft engine. Again no real accurate answer.

Why is this important?

The actual question concerns how much 100LL should be added to 87 R+M/2 mogas to satisfy the anti-knock requirements of a 91/96 octane certified aircraft engine.

And people wonder why I sometimes give non-specific answers to technical questions.

Using mogas in aviation has worked because there is enough margin between the 87 R+M/2 and the requirement of the 80/87 engines. This gives a margin of at least two or three octane on the lean side and, in these low compression engines, do not seem as critical on the rich rating requirement.

For higher compression engines, that margin of safety needs to be increased to be safe under all conditions and applications.

On a personal note, I was saddened to learn of the passing of an old friend, Bill Mangold. He was a great guy and helped build the general aviation business with his knowledge, wit, and ability to help people. He will be missed.