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?
I will totally agree with the use of an unleaded fuel for piston-powered aircrafts. As we all know that lead in the form of TEL is exposed to air and EPA has actually classified TEL as a group 2A carcinogen.
Back in Malaysia, i tested a Lycoming O-320 using marketly available unleaded fuels (u know what fuel i mean) and some blends with the AVGAS 100LL. I must note here that use of a fuel apart from the 100LL is strictly prohibited in Malaysia as per the rule set by CAAM in 1987!
I found that the detonation performance (anti-knocking ability) of a fuel with a lower octane rating is closely associated with the composition of that particular unleaded fuel. AVGAS 100LL is well-known for it’s high paraffin content and a very low aromatic content. While most unleaded fuels are high in aromatic and very low in paraffin content (i tested the fuels for conformation & it’s a YES). This is an important parameter of evaluation. An unleaded fuel having significantly high amount of Aromatic content is able to outperform the detonation performance of the AVGAS 100LL especially at speeds higher than 2000RPM.
Besides that, high aromatic content of a fuel gives considerably high emissions and this can be reduced by adding some olefins in the fuel.
It must also be noted here that, the RON rating of the AVGAS 100LL is 110. RON rating of a fuel can be closely associated with the engine design; the compression ratio. From literature, I found that, the compression ratio of engines produced by Lycoming fall below 9.7:1.
An engine with a compression ratio of 9.7:1 HAS NO necessity of using a 110 octane rated fuel. From the Octane vs Compression chart, we can approximately predict that fuel below the RON rating of 98 are sufficient but now below 92, are sufficient and produce optimum power, thermal efficiency, minimised fuel consumption and minimised emissions.
Hence, my conclusion is that, AVGAS 100LL is eventually causing us more fuel, less power, polluting the environment and making humans sick (one of the major source of lead emission in the world).
How about we simply switch to turbine engines on everything? Use of lead, octane concerns & etc will vanish & all we have to do is use some versions of kerosene. Military Tanks are using turbine engines & who would’ve imagined that, or, not laughed at the idea during the years prior to its introduction? Enough turbine engines will be sold to make it worth the efforts on the parts of the manufacturers, so it’s not like everything’s going to skyrocket in price. They’d put their selves out of business & out of jobs if they try to gouge us with the usual “R&D expenses recovery” excuses. When refineries of MoGas began producing unleaded, which costs them LESS to blend & deliver, they charged us MORE because it was new & catalytic converters required it. We can make sure turbine engine production doesn’t offer a gouge-opportunity to any of them in a number of ways. Just do it! Then enjoy flying TP-152’s & TP-172’s & more.
Even if the consumer doesn’t end up covering the R&D expenses, the liability is what brings the cost in aviation up most. I don’t know of anyone that thinks putting a turboprop in a <$30,000 airplane is feasible. Turboprop engines are less efficient than reciprocating, and as you make a turbine smaller the efficiency decreases further. Your range and payload would also decrease because of the heavier fuel despite the lighter engine.
Diesel engines in aviation would be more feasible, but they have another set of drawbacks,
I like this: “Why not go straight to a 94ish UL and leave 100LL alone?” By 94UL I hope you mean just 100LL with out the lead but still including the rest of the additive package plus what ever needs to be added to make up for no lead. That makes the most sense to me.
Excellent article, Ben
I do not understand the fervor chasing unleaded fuel for aviation. Given the recent fiasco in PAFI testing and now knowing 100LL may have to stay … what’s the point. SO little fuel — relatively speaking — gets used for piston powered aviation, just leave it all alone.
Thanikasalam Kumar, Rahmat Mohsin, Mohammad Fahmi Abdul Ghafir, Ilanchelvi Kumar, Ananth Manickam Wash,
2018, Concerns over use of leaded aviation gasoline (avgas) fuel, Chemical Engineering Transactions, 63, 181-186
DOI:10.3303/CET186303
For your reading pleasure Larry
Considering the EPA violated their own getting protocols in order to artificially increase the amount of airborne lead detected in the study that is used to justify removing leaded fuel from aviation, I have to agree with you Larry.
https://www.flyingmag.com/news/airport-disputes-epa-lead-air-study-findings
Excellent description of Octane number. So many pilots (and sports car owners) do not understand that number – simply thinking it is an indicator of power available from the fuel – and not comprehending the impact of low – medium – high compression engines and their need to prevent knocking – or pre-ignition of the fuel.
Next – take on Cetane! A completely different problem for all those wanting to fly with compression-ignition engines (diesel type) and simply use “truck fuel” – which in itself is a different subject (paraffin’s) – for aviation operations – and Jet Fuel is NOT diesel fuel – but it can run in diesels….
Ben, that was an incredibly informative article with a lot of information regarding the history of how the earlier aviation octane ratings were derived and how we ended up with where we are. I absolutely agree. We might as well be selling the 94UL, and wish the airports would go ahead and switch over now. No need to go to the extra expense to create the 100 LL replacement if it isn’t going to work. Most of us in the GA fleet don’t use it anyway, and the high lead content is detrimental to many engines.
Wish the government was capable of showing a bit of common sense and allow a much leaner lead content in the fuels. That would be good for the engines, and much less expensive to support vs installing a two tank system for two fuels, as most FBOs aren’t willing to put in more tanks due to the associated costs.