In the past few years, I have written a lot of columns about octane ratings. However, the questions I receive indicate there is still a lot of confusion as to what the ratings systems are and their relationship to each other.
Many people do not realize that the octane rating of a fuel is not a physical property of the fuel, like specific gravity or pour point. It is a performance perimeter that gives a rating for the fuel under very tightly controlled lab conditions.
How that fuel will perform in the real world under a variety of conditions is another matter.
The octane of a fuel was first rated using the research method.
In this method, iso-octane had a rating of 100 (thus the name) and the fuel component n-heptane had a rating of zero. To get a reference of 90 you would mix 90% iso-octane with 10% n-heptane.
The biggest problem with the research method is that it had a very poor correlation to knocking in the real world, for instance, in actual cars. So the fuels and lubricants committee for ASTM worked on the test and found that if they raised the RPM of the test engine and increased the intake air temperature, they got a much better correlation to real world engines. This became the motor method.
But the ratings were significantly lower, so the marketers refused to use it. They finally compromised and use the average of the two methods on the pumps.
In aviation, they started with a method called the lean rating. Then with the advent of supercharged engines and the fact that most aircraft operate on the rich side, they developed the rich or supercharged rating.
Having two rating systems for aviation fuel was expensive to support with the decline in fuel sales because of the increased use of jet engines. So they determined that for aviation alkylate based fuels, there was a correlation between the lean rating and the motor method. The use of the real lean rating was basically discontinued.
Two important points here. First is that the correlation of the lean rating to the motor method was only measured for alkylate based fuels. I have never seen any data that shows that the relationship is true for other types of fuel.
The second point is that the rich or supercharged rating is much more difficult to run than the lean rating test. However, no correlation between the rich rating and the motor method has ever been shown, even in the lab, let alone the real world.
In fact, history shows just the opposite. When the industry changed from 100/130 to 100/130 low lead, the average lean rating went up and the average rich rating went down.
So if the lean rating correlated well with the real world, then knock complaints should have gone down. But they went up very significantly in many types of aircraft. This would indicate that the rich rating is a better indicator of real world performance.
One interesting point about the rich rating is the reference fuels. If 100 octane is the high reference, how do you reference a fuel with a rating of 130? What they use is iso-octane plus so many grams of lead. 80/87 was usually just alkylate with a half gram of lead, which had an octane in the mid-90s, so the octane was not a concern.
The rich or supercharged method works well with leaded fuels on which it is referenced, but if the lead is removed, it does not correlate well. Much of this is due to something called the lead bonus.
This means that in the real world a leaded fuel will rate significantly higher than an unleaded fuel with the same lab-measured octane or anti-knock rating.
So when people say that an unleaded fuel with a lean rating of 100 will satisfy all of the 100LL aircraft in the fleet, they really do not understand the show.
The bottom line is if they come to market with a 100UL, they will still need to market 100LL.
So they will have two fuels, with the 100UL costing more and not performing as well as the 100LL. Why not go straight to a 94ish UL and leave 100LL alone?