With all the news about the development of unleaded avgas, some pilots may be concerned about the potential of exhaust valve recession when unleaded fuels are used in their aircraft engines.
That lead me to thinking that I should explain the background and some of the history behind the concerns about unleaded fuel and engines.
When I started working at Shell Research, it was 1967 and automotive emission controls were just beginning nation wide. (There were controls in 1966 and 1967 in California only.)
In addition, reduced future emission levels for the next 10 years or so had been outlined and it was apparent that exhaust catalytic converters would be necessary to meet these future standards. If this was true, unleaded fuels would be necessary because the lead in the exhaust would poison catalytic converters.
Additionally, in 1970, GM announced that all 1971 and later vehicles would be equipped with hardened exhaust valve seats and lower compression ratios so that they could run on unleaded fuel.
Exhaust valve recession on unleaded fuels was well documented in technical reports and other periodicals, so the oil companies were looking for remedies for the problem.
At Shell, the fuels research group was charged with developing an additive package that would eliminate the recession problem in older vehicles. The first step was to develop a repeatable test for exhaust valve recession.
We started with a 1968 430 cubic inch Buick V-8 engine mounted on a bench dynamometer test stand. We ran it on a relatively high speed, high load test cycle for more than 300 hours with no significant problems.
We reasoned that the engine probably had been run on leaded fuel at the factory, so we replaced the head and valves and repeated the test. We had some recession but it was not consistent.
On the third test, we replaced the fuel run tank, the fuel lines, pump and intake, and carburetor. On this test, we had recession on all of the exhaust valves at 200 hours — and we could repeat the test accurately.
We then ran a series of additives to determine their effectiveness. TCP was the most effective of the additives tested, but not nearly as effective as TEL, or Tetraethyl lead, the lead that is added to 100LL to prevent knocking.
So what happened to the additive program? Nothing.
The EPA talked about banning all leaded automotive fuels in the late 1970s, but a total ban did not come for more than 20 years. By then, most of the pre-1970 automobiles had been scrapped or overhauled. And the rebuild industry had switched over to hardened exhaust valve seats.
Once leaded auto fuel was completely gone, there were numerous recession failures, but they were in old vehicles, usually trucks, that needed replacement any way.
Surprisingly, gas-powered farm tractors did not have too many problems, primarily because of their low operating RPM.
What About General Aviation Aircraft?
So why won’t this be the story in general aviation piston engines?
First, air-cooled aircraft engines run the exhaust valve and seat at much higher temperatures than liquid-cooled automotive engines.
Second, aircraft engines run at much higher loads than automotive engines at cruise conditions.
Third, aircraft are not replaced as often as automobiles.
And, finally, aircraft engines already have hardened valve seats.
There are a number of factors that can affect exhaust valve recession, such as cam profile, valve face angle, coolant flow, carb setting, ignition timing, etc. This is why the problem does not just affect the large turbocharged engines.
But a large factor in this problem is how long will 100LL be around?
If 100LL is around for a while, then pilots can run a few tanks right after overhaul and the lead effect will help keep the exhaust valves safe.
But if the EPA tries to outlaw 100LL within a year or so, there may be a significant problem.
About 30 years ago, a California refiner noticed that the ASTM spec for 80/87 avgas listed a maximum lead level, but no minimum. So it started to market an unleaded 80/87 fuel.
Within a short time, it started to receive valve recession complaints on newly overhauled aircraft engines. The fuel was removed from the market.
I have also received numerous reports of recession problems from rebuilders on newly overhauled engines run on unleaded fuels. This is not a problem that occurs on every engine every time. It is just another thing to monitor, especially with a new or overhauled engine.
Having read all the comments about 100LL an the trouble it
causes in aircraft engines I personally found a solution I own a C-150
and I had troubles with sticking valves So I switched to 50/50 mix
of premiam gas and100LL worked for 500hrs later Try it
Not sure why the commotion. Automotive simply shifted to hardened seat, valves and guides, and retrofit is available for legacy. Not that my ’53 Cadillac and ’66 Ford truck haven’t done just fine with their original engines and using unleaded for decades.
Ben,
You confused me with “aircraft engines already have hardened seats”, then saying we need some 100LL on new or OH’d engines for the first few hours. I thought hardened seats negated the need for 100LL for valve seat recession.
This potential ban on 100LL really worries me. My husband and I bought a 1980 Mooney M20J in July 2020. During touch and go practice in summer 2021, I lost my engine on climb-out. Thankfully, we have a long runway and I was trained well how to handle this situation, or I wouldn’t be here to write this. It turns out, the problem was exhaust valves. We had them reamed and the engine is healthier now, but this is a scenario that I hope to not have to experience again. I am super-sensitive about engine issues (we’ve actually experienced four engine-outs), and the idea that the EPA would be so cavalier with human lives just astounds me. If you look at the overall picture, GA aircraft have an infinitesmal effect on the environment. Doing the “little things” like not disposing of sumped fuel on the ground and just properly maintaining our engines would lessen the effects even further. Thank you for publishing this article.
The EPA is “Cavalier with human lives”. The general aviation currently in the US currently emits 1 million pounds of lead into the atmosphere each year. They are responsible for over 70% of all new lead emissions in the US each year. Studies have shown that children living near GA airports have increased levels of lead in their blood. Does that astound you?
Mary Margaret McEachern – If your valve guides needed to be reamed to fix your engine after it failed, it was because you had a stuck exhaust valve DUE to LEAD build up in the valve guide and on the valve stems. Eliminating leaded fuel would have PREVENTED your engine failure. You should be more concerned that the FAA is leaving you with no choice but to run LEADED fuel.
What oil are you using?? I saw a lot of exhaust valve guide carbon build up in engines using other than Aeroshell oil. Lycoming has or had a service bulletin suggesting valve guide reaming at 500 hour intervals as I recall. To address carbon build up. Yes, I worked on GA aircraft for 35 + years.
So what is the answer for radial engines? What are we going to do to keep flying big round engine warbirds?
Bardahl made, and still makes, a product called Instead o’ Lead, which I used a long time ago in lawn mower fuel, and in my ’62 T-Bird 390 engine gas, when leaded fuel was scarce.
Instead o’ Lead can be gotten in two verities original and gold on ebay. The mix seems to be 4 oz to 6 gal. Didn’t find it elsewhere. Don’t known anything else except reading the warning pdfs.
Pilots are worrying about valve recession with the lack of lead, but in the mean time, they are destroying the valve stems and valve guides by running the huge amount of lead in 100LL. With the newer valves and hardened seats, you just don’t see the exchange of metal at the valve face that we did with the old soft seats. It would be great if we could buy a TEL additive we could add to our fuel for 25 hours during break-in as that would add significantly the life of the valve faces, but unfortunately, the government has an all or nothing attitude, and many consumers are foolish enough to add this stuff continuously thinking they are helping their engines. One of the most common misconceptions I hear from pilots is that they think TEL is used to “lubricate” exhaust valves. The fact is, it is there as to prevent detonation, but was also found to work well as a sacrificial metal on the valve face as the older valves and soft seats would exchange metal at high temps causing valve face recession. That problem was addressed with hardened seats and better metallurgy in the valves. Yes, there are still some older engines running with soft seats, but for the most part, any engine that has cylinders manufactured in the last 30 years should have hardened seats. And any engine with soft seats has had enough lead run through them to protect them through runout. So why are we still trying to hang on to leaded fuel? It causes premature valve guide wear/failure, and engine mechanics are chronically exposed to high levels of lead salts that is damaging to their health and mental well being.
Come on FAA. Get out of the way and let’s get the GAMI (and other) fuels certified and into production by the oil companies.
Thanks for pointing out that TEL does NOT “lubricate” exhaust valves….
Of course, that’s what Marvel Mystery Oil is for!
Ben,
I’ve had an exhaust valve recede into the seat about 1/10th inch on a 1,000 hr cylinder on my GO-300, [ in a Cessna 175B ]. It moved so far that the valve spring retainer was contacting the rocker arm, holding the valve open enough to show ‘0’ compression.
The valve hadn’t burned yet, but the cylinder was junk. [ overhauled once already ].
The engine was running fine; it was time for an oil change, and I always do a compression check while the cowl is off. [ now I’ll be doing borescopes of the cyls. ]
So, I wonder if I should save a few 50 gal. barrels of 100LL to use from time to time to extend the life of the exhaust seats ?