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.
I can only comment on unleaded fuels thru my automotive experience.
Tetraethyl lead (TEL) was an additive put in fuels from about the late 1930’s used to boost octane values for engines that had increased compression ratios (and hence, higher HP ratings). Check ‘History of the Ethyl Corporation’ on this). The use of TEL was effective, and was also used in aviation gasoline basically for military aircraft engines and the increase in power outputs from them. The Shell Oil Company, and Gen. Jimmy Doolittle were instrumental in the furtherance of TEL research and usage.
When auto engine compression ratios were 5:1 and 6:1, leaded fuel wasn’t necessary. They would run fine on ‘straight gas’, which was fuel that came straight from the refinery with no additives. With the advent of higher compression ratios, such as the 1949 Olds Rocket V-8 and its 7.5:1 ratio, higher octane fuel was needed, and leaded fuel fit the bill. This was also true for aircraft engines of the time, with their relatively low compression ratios, and low power-to-cubic inch displacement ratios.
The downside of leaded fuel was combustion chamber deposits and spark plug erosion. It was common then to replace plugs at 10,000 mile intervals (and today, it can be up to 100,000 miles).
To counter combustion chamber deposits, bromine was added to gasoline, which was to scavenge lead deposits. Not that it was all that effective, but it was better than nothing.
An upside to TEL use was that lead served not only to boost octane ratings, but also provided a measure of lubrication to valve faces and seats, limiting wear (or erosion) of these surfaces.
About 1975 was when unleaded fuel was mandated for auto engines, and toluene, among some other compounds was used to boost octane ratings in lieu of TEL. A problem I experienced was that these compounds, over time, would create a ‘cone’ of material on valve stems, between the lower portion of the valve guide and the valve face. This material was the consistency of very hard plastic, and was coal-black in color. This deposit caused a choking down of the intake area upstream of the face, which artificially leaned the incoming charge causing preignition and possible detonation. The only real fix was to remove the valves and clean off this material.
Of late, I have seen none of this build-up as had happened before….fuel additives have changed. Also, in the ’70’s, there was all this haranguing about valve seat recession. Hardened valve faces and seats, such as the use of stellite, were to alleviate this problem. In fact, it was later proven that day-to-day normal driving would not cause this recession, however having a consistent ‘lead foot’ or towing trailers, or operation at constant high speed would tend to wear valve faces and seats.
I feel that unleaded fuel usage of lower powered aircraft engines may not cause issues, but my question stands as to its use in high-output and turbocharged engines. No block test is going to show this……it will take time-in-service and calendar time to show what problems may result.
The P&W R-985s shown in the article are 80 octane engines. The 600 hp R-1340 is as well and most smaller radials Continental 670, Warner Scarabs, Kinners etc, are 80 or LOWER octane engines. The “A” series Continental was certified with 73 octane witch was an unleaded fuel Many of the big engines R1830 or larger already have operating instructions for operating on lower octane in there respective manuals.
Greg,
I would be curious to know if these engines would be susceptible “valve recession” due to unleaded gas? Or what makes a valve susceptible to recession. I also believe the DC-3 is approved for Mogas if I remember correctly.
The timing was reset to 24 degrees per AD 77-13-03 to prevent cracking on early designed O 200 cylinders of certain serial numbers.
I am curious what cause the valve recession with out lead. Other piston engines in the world are running with out lead and do not have this problem, so it must be the materials used in aircraft valves??
Engines are designed for specific fuels and other types of fuels may be harmful to the engine. The fuel can be designed for the engine, but that is rare and not very often achieved or even attempted.
It has been stated ad nauseam that gasoline with a higher octane rating burns slower than gasoline with a lower octane rating.
If this is true, has consideration ever been given to the possible need to change the ignition timing with the use of the gasoline with the lower octane rating? The thought being that the location of peak cylinder pressure would occur earlier than it otherwise would, with a consequent loss of power being developed.
In the mid to late 1970’s when 80/87 fuel was phased out, operators of small Continental engines were stuck with using 100 LL gas…..mogas wasn’t an option then. What I experienced as a mechanic with a flight school, the static compression of the cylinders in the. 0-200 engines in our C150 aircraft began to drop, most noticeable after about 200 hours of steady operation on 100 LL. TCM and Cessna later issued a service bulletin (may have been an AD….can’t recall) to change the static timing on O-200 engines from 28 to 24 degrees BTC. Perhaps this relates to about what you speak. Neverthess, retimed or not, it was commonplace to have to remove one or more cylinders and reseat or replace intake valves on a 100 hour inspection. This problem persisted, and likely does still persist on small TCM equipped aircraft that see moderate to high operating schedules.