In the past, most of my columns answered questions from our readers.
This time I am going to ask readers a question to answer — or at least think about: Why was a half gram of Tetraethyllead (TEL) added to each gallon of 80/87 avgas?
In 1947, the American Society for Testing and Materials, now known as ASTM, issued specification D-910 for three grades of avgas: 80/87, 100/130, and 115/145.
The physical specifications for the three were all about the same except for the amount of lead and the octane rating. This resulted in these three grades being blended with the same bottomed alkylate but with varying levels of lead.
For example, oil companies would take the aviation alkylate and add lead until it reached 100/130 octane or they would continue adding lead until it reached the 115/145 level.
Now for 80/87 fuel they would take the aviation alkylate and add just 0.5 grams per gallon. But why?
Typically, the aviation alkylates had an octane in the low 90s, which was well above the 80/87 octane specification. So why add any lead?
The interesting part of this is every manufacturer of 80/87 avgas added lead to their fuels from 1947 on except one.
In the 1980s, a refinery on the West Coast noted that the D-910 specification for 80/87 octane avgas only listed a maximum lead level of 0.5 grams per gallon, no minimum. Since it did not have any experts to tell them why it should not do it, the refinery started selling a zero-lead level 80/87 avgas.
A lot of people, as well as industry experts, may find this hard to believe, but the refinery got a whole bunch of exhaust valve recession complaints.
For example, a flight school in the LA area lost two O-200 engines out of Cessna 150s shortly after they were overhauled and returned to service.
There where many more claims until the refinery took the unleaded product off the market.
ASTM then revised the D-910 specification to close the zero-lead loop hole.
We need to analyze this experience carefully in light of our current transition to unleaded avgas.
Back then, there was a lot of research, which lead to expertise, concerning aviation piston engines and their fuel requirements. So if members of the ASTM committee agreed that a small amount of lead was needed in 80/87 avgas not for octane but for exhaust valve protection, we may want to take note of that fact.
Further, the fact that every one in the oil industry — except one refinery — bought TEL to add to avgas is very noteworthy. I worked in the oil industry for 35 years, and believe me, if the lead was not really necessary, it would have been dropped early on to save money.
General aviation is now facing a forced introduction of a 100-octane unleaded avgas in the future, with 2030 the target. Unfortunately, we no longer have the level of expertise that was present when avgas engines were the major source of power in the aviation world.
But just because people do not believe exhaust valve recession is a problem, that does not make it go away.
Ben, I always use TCP with 100LL in my 65 HP Continental. I fly this plane around 100 hrs per year w/o fuel related problems of any kind. Of course the TCP does exactly what you expect as far as clean plugs, my question does the lead still protect the valves from erosion?
After two articles claiming eminent death for all aircraft engines, I’m calling BS. The articles contain wild undocumented claims and unsubstantiated warnings of peril. We made it just fine through the change in automobiles from “regular” to “unleaded” and now it seems like a distant memory. I ran a 1964 Olds 425 engine on unleaded gas season after season of full-load hard pulling in my jet boat and never once experienced valve recession. Valves and cylinder heads are wear items anyway. Most aircraft are lucky to make it to TBO before needing a new cylinder or two. Oh I’m sure there are some one-off cases where someones bird will have some valve recession and unleaded fuel will undoubtedly get the blame, but most folks will expire of old age before unleaded fuel causes any issue. Many Cessna aircraft have the auto fuel STC. Guess what, not a problem at all. Folks have been running unleaded in STC’d engines for years now. Personally I welcome the change and look forward to no longer worrying about absorbing lead in my skin when I sump my tanks. And lets be honest, it’s coming, might as well embrace it. Lets all worry about something more substantial, like where is the best fly-in hamburger, or where did all my wife’s Tupperware lids go?
Hello Ben,
I think some of your history is inaccurate. Although unleaded 80/87 was produced, the gasoline blending pool looked different back in the late 70’s and 80’s. With the stepped phaseout of lead from the mogas pool, folks were running their catalytic gasoline reformers at higher and higher resultant octane (whether called Platforming or Rheniforming (your company and mine) or Powerforming or Magnaforming or Ultraforming or Houdriforming or Octanizing). That resulted in light reformate that was ideal for avgas blending, particularly for 80/87. With the imposition of mogas oxygenate requirements, that opportunity went away as reformer octanes were reduced, and reformate was no longer added to avgas.
I don’t see the historical record requiring a minimum lead level in other than 100 grade; today’s version of D910 has no minimum lead level for 91 octane leaded avgas, for example. You also overlooked the other 1947 and later grades of 91/98 (later 91/96), 108/135, and 115/145, the latter of which are of interest to the warbird crowd.
In any case, the same apparent valve seat problem occurred with lead phasedown of avgas as had earlier occurred with mogas, as documented in relevant SAE papers. The valve distress was related to the reduction in *delivered* octane rating, not the lack of lead per se. The FAA validated this in aircraft engines beginning way back in 1989 by parallel runs of leaded and unleaded gasoline. GAMI has a link to this study on their website.
https://tinyurl.com/lead-vs-unleaded
Later, the FAA ran (on a twin) two carefully matched engines running carefully octane-rating-matched 100LL and unleaded prototype avgas. Again, there was no difference in valve seat wear.
One can correlate the shift from leaded to unleaded with engine distress, but once studies are carefully controlled for as-burned octane rating, the lead factor goes away.
The good news is that GAMI’s G100UL has an excess of octane, coming in at 100/160+ lean rating/rich rating. GAMI reports that over a decade of testing shows no valve seat distress. There have been paint-attack concerns from aircraft leaks (not unknown in the fleet) but GAMI reported at OSH they’re working on mitigations. I remain optimistic.
In the comments, Ted asks, “Why the FAA can’t stick six different but representative engines on test stands.” The FAA has done similar work (though not on 80/87 as Ted suggests). If you control for octane rating, engines do fine. The Swift Fuels 94 problems at UND are likely related to that same phenomenon: a reduction in delivered octane rating from 100LL to 94, and UND’s operation of 91/96-approved engines at peak EGT, a type of operation that was largely unknown before 91/96 was phased out in the early 1960’s.
Paul
Please explain to me why the FAA can’t stick six different but representative engines on test stands and run them on zero lead 80/87 for 2000 continuous hours (1 TBO), and then tear them down and see if there is any valve rescission?
1 month to set up, 3 months to run, 1 month to tear down. We would know inside 6 months.
This isn’t hard.
Ted
I went to Oshkosh. I listened to the Swift unleaded fuel guys. They said everything‘s great. so asked how many hours on the longest engine. They said 400 hours. Not convinced at all. They haven’t even run an up to 2000 hours.
Swift has tested there UL100 for 400 hours on an engine that is approved to use UL91. What exactly is it they are trying to prove.
I also am using the TCP with every fill up in my Continental C-85. For only about 25 cents per gallon of fuel, it is worth the effort. The engine has been recently overhauled, so it is worth more than the airframe.
Yes, even with the stuff costing $100 per qt, delivered, it works out to $0.31 per gallon.
Well worth the small added cost, with avgas here at $5,60.!!
Paul Bertorelli did a deep dive into this.
https://m.youtube.com/watch?v=ovJBJjZTjsk&pp=ygUcTGVhZGVkIGF2Z2FzIHRyYXNoIG15IHZhbHZlcw%3D%3D
As an addendum to my comment at my airport in Los Angeles, local government in response to community pressure spent over a million dollars to assess the lead levels at the airport. The results were not what they expected. Ambient levels at the airport were no higher than the rest of the county and actually lower than some areas.
There is so much lead left over from all the lead cars used over so many years I don’t know how they could detect any increase. They say that the lead pollution caused by car exhaust was so bad it has probably resulted in a 7 point decrease in IQ.
There are people way smarter than me working on this. The level of knowledge is on display in the comments and I appreciate the effort to protect my valves and hope we get rid of the lead. What puzzles me is this. We’ve had car gas in select engines for years and I am not aware of problems being reported. Is there an explanation or do we just not hear of them?
My Rotax loves unleaded. Obviously it can be done and has been done in cars without problem. As I understand it, the solution is valves designed for that type fuel.
Same problem with BMW motorcycles pre-1968 or 1969. Unleaded gas causes valve seat recession, and fairly quickly. These bikes quite normally ran 100,000+ miles (astonishing for a motorcycle) on regular leaded gas, problems arose quickly with unleaded.
BMW went to different valves (added valve rotators) in late 68, but that didn’t help.
There was at least one company installing hardened valve seats and tougher valves, but by now this is pretty academic as most of these bikes are now trailer queens and not driven every day.
Lycoming, knowing that unleaded fuel was on the horizon, started a program to change their valve seats to a type that resisted valve seat recession as they knew it would be a problem in the future after the change to unleaded fuels. Automobile engines already have gone through this transition and of course today we see those engines lasting twice as long as compared to the Lead Fuel designed engines.
I fly a Cessna 175B, with the GO-300 engine that I operate at 2,900 to 3,000 rpm.
With about 350 hours on the new cylinders I got 3 stuck/sticking exhaust valves, 1,3, and 5.
It is using a qt of oil per 14 hours, so the lead in the oil jumped to 3,000 ppm.
I had to ream the valve guides. Now about 300 hrs later, it has 1 or 2 valves having ‘morning sickness’, with reduced oil changes at 25-30 hrs.!
I have recently added 1 oz of TCP per gallon and after 4 hours, there is no more valve sticking, and the engine now idles smoothly at 700 rpm…it never did before.
So, scavenging the lead oxide/bromide to lead phosphate seems to be working to remove the lead deposits.
One of the early removed cylinders that had about 500 hr on it had the exhaust seat so eroded that the rocker arm was contacting the spring retainer, beating a divot in the lower part of the rocker arm….these are very expensive parts, that are not normally replaced.!!!
The engine has always be run on 100LL, with 2,400 hrs TT now.
So, I have no idea how the engine will run on 100UL ?
typo….The TCP is 1 oz per 10 gallons….
While Engine Components was still in business, we saw a number of valve seat recession problems. When removed, the valve seats looked like they had been machined or ground in the shape of the valve face. The engines operated OK until the hydraulic lifters could not compensate. We found out that all our problems were coming from operators using autogas.
Most aircraft piston engine exhaust valve seats were made from AMS 5700 steel. I am told that there is a high temperature oxide film that forms on steels with relative high nickel. I am also told that the film absorbs lead and creates a dielectric that helps prevent microwelding between the valve and seat. AMS 5700 steel was becoming difficult to obtain, and the L.E. Jones company came up with a cast version of that material (with some chemistry adjustments to help in the casting process). Serendipity occurred and the valve seat recession problem seemed to be behind us. With that being said, it was probably a delicate balance, and new fuel mixtures could exacerbate the problem.