A few years ago, I was giving a talk during a state aviation maintenance symposium when someone in the audience asked if there where any potential problems associated with the new proposed unleaded 100 octane avgas.
I explained that there could be a problem with an adequate rich knock rating for the fuel, which could lead to knock complaints in large engines and possible exhaust valve recession on some certified aircraft engines.
At that point someone else raised his hand and stated that he read on the internet that exhaust valve recession was a complete myth and there was absolutely no risk of this occurring.
I then pointed out that in the 1980s an oil company in California looked over the ASTM D-910 specification for 80/87 avgas and noted that the spec only listed a maximum lead level of 0.5 grams per gallon and no minimum level. It then started selling an unleaded 80/87 avgas. There were no problems for a while, but then there was a rash of engine failures because of exhaust valve recession.
Many of these problems occurred in engines such as the Continental O-200 engine powering Cessna 150s used in flight training operations.
A well-known engine rebuilder from the Twin Cities then got up and shared several slides of engines that he had worked on that failed after using unleaded auto gas and had experienced exhaust valve recession.
Following this information, the person in the audience who said exhaust valve recession was a myth said maybe the engine rebuilder was right, but he could not believe that because the article he read was on the internet, so it had to be true.
Since that time, I have written several articles about the possibilities of exhaust valve recession once all of the lead is deleted from aviation fuel. These articles had a fair number of comments, including some from people who did not believe that it could happen. I also talked to engine manufacturers who felt that it was not a concern.
And then the University of North Dakota, which has a large well-maintained fleet of aircraft, started running on unleaded Swift UL94 fuel and guess what? They started seeing exhaust valve recession on several aircraft.
So why is this happening at flight schools and not in private aircraft operated on Swift fuel or unleaded auto gas?
The big difference is the single source of fuel.
Many private aircraft were broken in on 100LL, plus many are used on cross-country flights, where they are refueled with 100LL occasionally. Most flight schools do all of their refueling at only one location, so most aircraft in a school’s fleet never see any lead in normal operation.
The reaction to the UND valve recession problem was interesting. Lycoming thought it could be due to the aromatic content of the Swift fuel.
But guess what? Most 100LL blends have contained aromatics since the early 1970s.
The only difference between 100LL and the Swift fuel is — wait for it — lead.
What are the main factors that prevent exhaust valve recession?
In the 1970s auto manufacturers found that hardened exhaust valve seats in liquid-cooled engines equipped with knock sensors helped prevent the problem.
The problem was also associated with load and the RPM that the engines experienced.
The problem for general aviation is that aircraft operate at high load, high RPM, with air cooling, and high exhaust valve and seat temperature — all factors that are in the danger zone.
Some of the discussion about the problem at UND included the thought that the university’s planes were operated on the lean side of stoichiometric, which would aggravate the problem.
But I believe they have multi-point temperature probes on all cylinders and they lean based on the monitor.
There is also a very real possibility for knocking on these engines using the 94 octane Swift fuel. Almost any knocking will raise the temperature of the exhaust valve and seat significantly and can lead to exhaust valve recession.
A multi-point EGT or CHT system may be mandatory for all aircraft before they are operated on an unleaded fuel.
In addition, knock sensors would help, but are not too practical in the aviation world because of individual cylinder assemblies.
What else needs to be done before we switch over to all unleaded fuel?
There are possible engine modifications, such as liquid cooling, but that would be a very high cost for every general aviation airplane out there.
I think the best answer would be to put an additive in all unleaded fuels to reduce or eliminate the problem. Something like MMT — Methylcyclopentadienyl manganese tricarbonyl, a fuel octane enhancer produced by Afton Chemical Corporation — might work.
The problem with MMT is that at normal treat levels the additive produces dark red whisker deposits on the spark plugs and other surfaces. These deposits can cause fouling and other problems.
But at very low levels it may not cause any problems and may prevent the recession problem.
What needs to happen to move forward towards an unleaded future for general aviation?
A repeatable valve recession test to determine if MMT or other additives — such as TCP, an FAA-approved aftermarket product that helps prevents lead fouling on valves and spark plugs — will eliminate possible exhaust valve recession.
But like lead and other fuel additives, TCP has its own health problems, including irritation of the eyes, skin and respiratory tract, drowsiness or dizziness, and depression of the central nervous system.
Is the unleaded fuel stable? If so, for how long. I feel like most any unleaded fuel now last a month or two tops in my shop.
Valve recession in air cooled motorcycles running on unleaded fuels has been solved with the correct selection of valve and valve seat material. A drop in fix. The motors are way more highly stressed than airplane motors. Granted that on a certified GA motor would require certification by the manufacturer would will not come cheap.
Well as a air-cooled VW buff and GA buff as well I know this…We all went to harder valves and seats and the old busses,bugs,gias and things work just fine..It’s not a lot of $$ and we drive Everywhere with these..The engines are very similar..I think that’s the answer but you can still add lead type additive to the fuel if you really must stay all stock..Bill VT
Nothing scientific is being conveyed in the above article. It is conjecture. Hardened valve seats seem to be the cure for the conjecture and the liquid cooling in a car engine is not by the valve seats, so that is also conjecture. Other air-cooled engines (motorcyles) have been running on unleaded fuels for years now with no issues of valve recession, and they run under a lot more extremes than an airplane engine. The main reason for lead is for octane boost and we now know that it is not needed to boost octane, because there are other sources for that.
When it came down to the Covid pandemic, it was said to follow the science. Now we know that there was no science applied to it and that it boiled down to the politics. Please don’t do this with coming up with an unleaded avgas. One organization has already developed a fuel for all aircraft to use, but it’s unacceptable, because they went the STC route instead of the ASTM gods. How ridiculous!
The next step requires wider testing of the high octane unleaded fuels available. GAMI’s fuel has superb lean and rich octane ratings. “IF” we still see valve seat problems, it’s probably not octane related. GAMI says: “that G100UL is superior even to the old 115/145 “Purple” avgas used on DC-7’s and Constellations”.
I can make an educated guess that some valve and seat problems will remain with all unleaded choices. Many other types of high load engines (NASCAR + 4 stroke motocross bikes come to mind) have been forced to work through the valve seat and valve problems by using exotic materials, even when using extremely high octane unleaded. It is not just a matter of “Hardened” seats. Many racers have moved to beryllium copper seats + coated TI valves
A leaded gasoline leak onto pavements and soil represents a KNOWN long-term ‘persistent’ environmental poison/toxin. The long-term human heath hazards… especially to animals and young children are well documented and very sad… and are totally avoidable by ‘0’ lead in the fuels. The hydrocarbon hazards are far-less internally persistent than LEAD. OH, yeahs… and the spill-clean-up process for leaded fuels can be grossly expensive to mitigate.
ALSO…
Lead in AVgas represents a unique challenge for distribution… it demands solely dedicated pipelines, above/underground tanks, fuel trucks and pumping equipment. Under NO circumstance can leaded fuel be distributed/dispensed otherwise. The potential for even the tiniest amount [trace] of lead cross contamination of unleaded auto-gasoline… will damage most, if not all, catalytic converters… degrading/damaging engine performance… Etc. AND the subsequent liability for this contamination event would be huge.
NOTE1. A ‘similar-to’ incident occurred in the city/region where I live, a few years ago. Raw gasoline was distributed thru pipelines to the local reception/distribution center [terminal] where alcohol and additives were added in ratios that are… were supposed-to-be… suitable for the regional climate, altitude, local-laws, etc… just prior to tanker-transport/distribution to the gas-stations. Somehow a bulk-load of gasoline was infiltrated with NOT 10% alcohol… but +20% [almost 25%] alcohol. The effect was punishing on autos that received the fuel and drove off ‘fat-dumb-and-happy’: severely damaged engines, accessories/pumps, hoses, etc… and very-least… contaminated tanks/piping/etc…. and god knows what else. OH yeah… and contaminated gas stations were-force-shut-down and had to be were fully purged of the contaminated gasoline from the bottom of the tanks to the fueling nozzles.. What a mess. Thank god… that I did NOT buy any gas that the couple of weeks when this happened.
NOTE2. During the Vietnam war, contaminated JP4 jet-fuel entered the SVN pipeline system and caused the loss of many aircraft… and lost lives… due to contamination and sudden engine starvation… before the catastrophic contamination was identified in BOLD LETTERS!! I cannot say anymore about this incident… since it was classified a Top Secret at that time. The details I learned were unclassified reports years later. NOT 100% sure that ‘cat’ should have been ‘let-out-the-sack’…
Why is there no discussion about GAMI fuel in the north dokots fleet ?? It has been fully approved by the FAA ? Whats up, and it’s 100 octane not 94 octane??
Bill sutton
Is there a reason that ethanol cannot be added to aviation fuel? It is readily available and inexpensive. It’s a cheap octane enhancer. It works in automotive fuel. I have a jet ski ( I know the engines are different) that I run E85. Running E85 allows me to advance the ignition timing for more performance.
There are a number of reasons that ethanol is not allowed in aviation fuel;
– the Mogas STC specifies ‘No Ethanol’
– ethanol is corrosive to aluminum
– the fuel tanks, fuel lines, fuel selector valve and carburetor are all aluminum.
– at altitude ethanol will become a vapor stopping fuel flow to the engine.
Also, ethanol has about 76k btu per gallon vs avgas at 114k btu per gallon, so much higher fuel consumption and reduced flight range.
so, not acceptable.!
Life was simple when you could still get all four flavors of Avgas: Red, Blue, Green…and Purple.
Is it time for the rotary valve to come into favor? Seems like a good thing to dig into.
https://www.hagerty.com/media/maintenance-and-tech/is-this-the-next-great-leap-for-internal-combustion/
New design innovation is great.! We can always utilize new engines.
BUT, there are over 250,000 existing engines in aircraft that need to be accommodated .
Retrofitting them with exhaust valve seats or new cylinders is a huge cost, essentially a full top overhaul.
Is there any study/ testing to determine the minimum amout of TEL needed to maintain the octane and the exhaust valves. Obviously 100LL still has too much TEL for environmental concerns.
We all want what we want, we have aversion to change and the need for it, but with out change and science we won’t be able to fly, have electric cars (controversial) or go to the moon, the average person can’t afford to have a private plane or hobby in flying, soo I guess it all comes down to a want or need, we all need clean water, air and a better environment then what we have created, I love flying and old cars
Flight school I was involved with put a dozen sets of aftermarket cylinders on their 150s and ran them on unleaded car gas (under the EAA STC). We had failures and had to replace them. The manufacturer blamed the car gas. The failures were valve seat recession.
Pre-1970 BMW motorcycles (air cooled flat twin engines, 100,000 miles was not unusual and these were well engineered motors made of quality components) had exactly the same problem. A few thousand miles of running on unleaded gas was all it took – valve seat recession, valves sticking in guides, same problems as our aircraft engines.
The CURE was a set of hardened valve seats, new valves made of a different alloy, and the exhaust valves had valve rotators (change in the keepers and collars). A couple of hundred dollars in parts and a trip to the machine shop fixed it for good.
Since it appears we are going to get unleaded gas like it or not, perhaps the same cure might be applicable? Harder valve seats, different valves, and a few extra bits and our engines will run on whatever swill we are obligated to feed them? If I had to spend a grand or two (and in aircraft, nowadays that is almost sofa money) to never have to worry about this again, I’d grit my teeth, but I’d do it.
Vintage auto engines are retro fitted with hardened valve seats and it seems to work for them. Couldn’t something similar be engineered for GA?
What is the effect of adding 1 Oz of aviation motor oil to 30 gallons of unleaded. It should reduce knock and lubricate upper cylinders for longer engine life.
Adding oil to the fuel will reduce the octane rating, and will increase the possibility of detonation.!
Aircraft engines, being air cooled, have more clearance between the piston and cylinder. So, they burn oil that is left on the cylinder walls, about 3 to10 hours per quart.
So, we have to check the oil level before every flight.
Btw: Maybe UND has a chemistry dept
that can formulate a magic elixir.
How about ‘reduced lead’ instead of no lead?
They seem to have the fleet and skills to help us,plus they’re on our side unlike do good beaurocrats.
Crashed aircraft cause lots of pollution too.
Good motors use rotary valves !
Says so on the internet !!!
I think we have lost sight of the initial process, It was my understanding that , and possibly so that this was all tied to reducing the carbon footprint with huge airlines burning hundreds of thousands of gallons of jet A on a regular basis, this possibly is an issue. But we then have had a Small number of people with an Agenda trickle down this nonsense to General Aviation, totally different fuel and engines in the GA Fleet and a Minimal if any real threat to the Environment. It seems to me we have let a small group of people turn a mole hill into a Mountain. Just my Thoughts, I like to think our opinion still matters as much as the fringe elements opinions.
Let us make a 1000-page proposal to the powers that be, that in 10 years we will decrease lead emissions 50% by reducing lead in our fuels by 5% per year. Tie this directly to other systems that need changing like the percentage of fluoride in our water, the percentage of Mercury in silver fillings, Thimerisol in vaccines, and the relative age of all congressional members with an algorithm derived by an AI. Since nothing will ever change along those avenues, we should be able to continue with the current fuel mixture forever!
It’s an easy problem to solve – Leave it alone. Continue to burn 100LL and forget about the entire unleaded fantasy. The “fantasy” being that we will somehow save the environment by switching. GA’s effect on the environment is insignificant at best. Our old (100 yr old technology), air-cooled piston jobs were made to run on high octane leaded fuel. Who is going to pay for all of the damages?? The owner and possibly the their passengers with their lives.
You are undoubtedly right that it’s negligible. The problem is that the tiny concentration that is measurable is around airports and the ecoterrorists who live near those places use it to claim the sky is falling. You will have a hard time convincing eco-zealots it isn’t an actual problem.
Mr. Visser, thank you for staying involved on this topic in your retirement. Your wisdom is rare in aviation these days. One question – has anyone looked into the effect of widespread mogas use in the Rotax series of engines? Mogas is widely available at European GA airfields where the Rotax is one of the most common aircraft powerplants. They seem to have licked any lingering concerns about using lead-free, ethanol-free mogas in aircraft engines.
Rotax engines are water cooled, for the most part. Not a good comparison.
As already pointed out, the Rotax engines that most people think of are largely liquid-cooled. And, as it happens, those engines also tend to have the knock sensors mentioned in the article, which are utilized by their digital engine management systems. Basically, like car engines, the Rotax engines leverage computer technology to compensate for wider operating conditions, while delivering good power for displacement AND safely avoiding operating parameters that could cause damage. So, they make a good case for what could be done with technology.
However, as has been shown for essentially all of GA history, that level of sophistication and complexity just isn’t *required* most of the time, meaning there has been little motivation to adopt FADECs and the like. At the same time (and, unfortunately, in my opinion) the same strict, labyrinthian body of regulations that Congress and the FAA have created to make aviation “safe” have made it incredibly expensive and difficult for manufacturers (and aircraft owners) to “lean forward” by adopting new technology in preparation for an eventual switch to unleaded. As a result, I’m concerned that GA in the USA may be flying toward a political, social, and bureaucratic mountain that it no longer has time to climb over.
DITTO! Thanks, Ben, for ‘schooling’ us all.
“ The only difference between 100LL and the Swift fuel is — wait for it — lead.”
Well, no. The Swift fuel also has less octane (94 vs 100). So now you have two variables in the test.
So was the problem with valve recession due to the lack of lead, or the lack of octane?
I hear that the UL 94 burns a little faster than the exact same fuel with the lead additive.
With that in mind I would think that a more retarded ignition timing might be an appropriate fix for valve recession and knock avoidance when using UL 94. A knock sensor is designed to retard the timing when it discovers a knocking condition.
Ben,
In California the lead was replaced with MTBE for a few years, until it was found to be water soluble, when a leaking fuel tank in rural CA contaminated the wells .
Since most avgas is in above ground tanks with spill pans, is this a possible alternative to TEL.?
Since knocking seems to be required to get valve seat recession, educating pilots to avoid power levels at inappropriate mixture settings could be a solution to the valve recession problem. For example, full throttle at full rich for climb is ok, 65% power cruise with lean just above rough is ok, and let the aircraft operating handbook fill in the other power levels vs fuel flow.
In the long run the planes need to automate the fuel flow to match operating parameters of the airplane.
Read it on the internet… LOL!
Yup, there are some very foolish peepul out there.
Oh, by the way, exactly what is the problem with having lead in piston aviation engines ?
Is it the cause of … exactly what ?
Fwiw, I just read autism is being diagnosed for something like 1 in 9 kids these days, mostly boys. Sayonara fusa at that level. And it certainly isn’t the biggest problem this radically transformed country is staring down the barrel of now.
But get the lead out … yeah, that will really make a difference for the really big problem …