From my Feb. 23 column “Fuels: What to expect in 2017,” I received a number of responses, both positive and negative. I appreciate readers taking time to write back, even if they disagree with what I wrote.
The two areas I would like to address are the belief that all 100 lean rating fuels are equal and that exhaust valve recession is a myth.
Claiming that any fuel with a 100 lean rating will provide the same anti-knock protection in every aviation application in the real world, is just NOT TRUE.
This is like saying that every 200-pound man would be equal to every other 200-pound man in a tug of war. I believe that a 200-pound athlete would be able to out-pull a 200-pound couch potato who lives on donuts and junk food.
The two men are equal when they stand on a scale, but perform differently in the field.
In the same way, many fuels will perform the same in a well-controlled lab CFR test engine, but perform differently in real world aircraft engines.
Knocking in the real world is affected by a long list of variables, such as compression ratio, combustion chamber design, head temperature, air temperature, air barometric pressure, and on and on.
In addition, there is a thing called a lead bonus, which results in leaded fuels out-performing in the field any unleaded fuels with equal octane ratings. The best real world data for this was mentioned in numerous feedback notes, and that was when 100/130LL replaced 100/130 high lead fuel.
When an oil company blended 100/130 avgas, it would take aviation alkylate and then add lead until it reached 100 lean rating. It would then measure the rich rating, which was almost always well over 130.
When the company blended 100/130LL, it would add 2 grams/gallon lead to the alkylate, and then add toluene concentrate to meet the octane targets.
But now the rich rating was usually the controlling parameter.
The blender would keep adding toluene until it reached the 130 rich rating and then the lean rating was almost always well over the 100 mark.
The bottom line is the lean rating usually went UP 2 to 5 numbers, but in the field the knock complaints went up significantly. Let me repeat, HIGHER lean rating fuels had MORE knock complaints.
This is confirmed by many tests in the development process for an unleaded avgas.
For example, the people at GAMI have a 96 lean rating candidate fuel that ran with significantly less knock in their aircraft engine test bed then another candidate with 102 lean rating.
They, of course, could not market the 96 rating fuel because the GA public thinks that they need at least 100 lean rating fuels.The second area I want to address is exhaust valve recession with unleaded fuels.
This is another real world problem that will get significantly worse if 100LL goes away completely.
At the present time, if a new or overhauled engine is run on the dyno with leaded fuel and the engine gets a little 100LL every once in a while, the exhaust valves may be safe.
According to several rebuilders I have talked to, they are already seeing some cases of valve recession in engines that are field overhauled and then started right off with mogas.
Unfortunately, if some guy buys an engine that has been run on a dyno with 100LL and then gets a little 100LL during the life of the engine run mainly on unleaded mogas, and then goes to full TBO, he can write an article on the internet that claims valve recession is a myth, and people believe him.
In the real world, not every engine is going to have valve recession, but if lead disappears from the system, the number of cases of engine failure due to exhaust valve recession will rise.
If 100LL disappears tomorrow, about 85% to 90% of the fleet will notice only minor changes, like poor starting, different smells, etc. But the remaining 10% to 15% or so may notice pinging or knocking under some conditions. This will necessitate detuning or de-rating the aircraft.
In some cases, like with big radials, which may or may not be approved to operate on the new fuel, they may have to be de-rated to the point that they no longer are economically viable for their intended service.
Long term, as the lead is flushed out of the fuel handling system and people overhaul their engines, the cases of valve recession will rise. This may not happen to everyone every time.
But if your engine needs new cylinders after only a 100 hours or so, you may wish for the good old days when 100LL was still available.