by John Deakin
Tetraethyl lead has been gone from automobile gasoline for two decades, and it's only a matter of time before leaded avgas goes away as well. Despite a huge amount of industry research, nobody yet has a suitable replacement fuel, and nobody's yet quite sure what will happen to today's piston-powered fleet when the supply of 100LL dries up. AVweb's John Deakin dispels a bunch of myths about TEL, explains what it does and why it's so indispensable in high-performance recips, and talks about one solution to the coming unleaded-avgas crisis that actually works.
It's long past time to get out of the "Alice in Wonderland" mode on leaded fuels. There is so much misinformation out there on this, and so many OWTs (Old Wives' Tales), I hardly know where to begin.
There is a most elegant solution to the fuels problem now emerging, but more on that later.
First, a few of the OWTs...
- Lead does NOT "cushion" or "lubricate" valves. There is nothing in the serious literature, and no known scientific data to support this notion. If you know of something, please write. I said DATA, not some mechanic or overhauler mouthing this decades-old gossip. Remember, EVERYONE once "knew" the world was flat (and some still believe it). I do have one report that high-output marine engines had some valve problems when switched from mogas to unleaded fuels, but that same report said that hardened valve seats took care of that problem.
- Lead has no effect on the BTUs or "power" in the fuel. "Octane rating" has very little effect on BTUs or "power," and what effect it has is in the wrong direction. The more the fuel components are processed to raise the octane, the less "power" the fuel will produce. But the effect is almost negligible. The same "power" is generated by a gallon of 80/87, 91/96, 100LL, or the old 115/145, with or without lead. (What DOES change is the required timing for different fuels. Hold that thought. Timing is that all-important measure of when the spark lights off in relation to top-dead-center piston position.)
- Lead does NOT cause valves to run cooler or hotter. Lead does NOT prevent (or cause) "valve recession."
- Finally, EGT has little or no effect on valve temperature. Most of the heat comes from the very high combustion temperatures during the combustion event, 3,000 degrees Fahrenheit or more, and most of that heat is transferred to the cylinder, and the cooling fins. EGT shows the temperature of exhaust gas after the exhaust valve opens, by which time a great deal of the heat has been dissipated.
I know of NO known reliable DATA to support any of these notions, other than the well-meaning folks who happen to believe them, having heard them repeated over and over for decades. As I mentioned, there is some good data that refutes them, if you'll look.
Just for one example, the FAA ran a twin with flat sixes for several hundred hours, one engine running 100LL, and the other side with unleaded fuel. Then they tore both engines down and used some custom-built instrumentation to measure the valve wear. The lead did no good, at all. That's pretty good data, but I'd like to see more of it. Well done, FAA!
If any of you have scientific evidence for any of these, PLEASE point it out. I am NOT talking about "My mechanic says," or "everyone knows," I'm talking about published, scientific data by reputable people, who know what they're talking about.
At this point, let me point out that in the 30s and 40s, all the best and the brightest engineers were working with recips, and virtually everything we know about them comes from that era. Those people were ENGINEERS in the classic mold, and knew their stuff. What they did with primitive tools is astounding, and just about everything we're re-learning today was known then! Starting in the 50s, all the "best and brightest" gravitated to turbines, and it wasn't long before the general attitude was, "recips are dead." The old knowledge and "corporate memory" died, the marketing folks took over at Lycoming and Continental, and today we see the results. There may be a few real engineers left, but I don't see much evidence of that at the factories. The result is that most technical information coming from "the big two" is either intellectually dishonest, or solidly based on pure ignorance. There is NO curiosity left, and warranties are often worthless. The basic engines are usually excellent, having been originally designed by the engineers of yesteryear, but what the factories have done to them is pathetic.
Back to fuels. I do believe lead is bad stuff. It is toxic, known to be harmful to the environment, can cause fouling in an engine, and is expensive, adding a LOT to the cost of a gallon of fuel, especially when you factor in the transportation cost premium. Leaded fuels require dedicated trucks and pipelines for transportation to keep from contaminating unleaded car gas.
Lead is not "good" for an engine, IT IS HARMFUL to engines. But if you want high performance from an engine, it is the least toxic of the presently known chemical additives that will allow high power without detonation.
Folks, believe it. LEAD IN AVGAS IS GOING AWAY and well it should. Deal with it. It's already unavailable at any price in parts of Europe and Africa, and that is spreading.
If lead doesn't add power, what does it do? Like "octane," lead ALLOWS a higher power SETTING without detonation. The primary means of accomplishing this is in simply reducing the speed of combustion. By reducing the speed of the flame front, it takes longer to reach maximum pressure, and thus that max pressure occurs later, after top dead center. Can you spell T-I-M-I-N-G, again? (For more on detonation, see my "Detonation Myths" column.)
As I understand it, that is not all it does with respect to detonation, but it is the primary observable effect.
When lead is needed, there is NO known practical (chemical) substitute, and none is likely to be found. The big petroleum companies have MAJOR reasons to find a fix (cost), and they and the FAA have been searching and experimenting very hard for nearly 10 years, at great expense. With no results. I've been to a lot of seminars where representatives from the petroleum companies stand up and expound for an hour, when they could have saved a lot of time by standing up and simply saying, "There has been no progress in this area," and sitting down again.
Well, as one friend just pointed out, that's a bit harsh. Charles F. Kettering and his assistants tried about 40,000 different combinations before he struck upon gold … er, lead as an anti-knock additive in December, 1921. Each of those trials was a "failure," but each of them also eliminated substances, which is a "success" of sorts. No one at the oil companies is going to expend that kind of effort for a marginal improvement on a fuel (avgas) that represents less than 1% of the total gasoline pool.
They are NO closer to a solution than they were when they started. There is no "magic bullet" here! Yes, there are other chemicals that will do some of the same things as TEL, but all are more toxic, all are more expensive, and none do the job quite as well. Switching from lead to any of the other known solutions will ground a huge number of aircraft, which I find an unacceptable "solution."
Even if some chemical compromise could be found, getting FAA certification for current piston-powered airplanes to use it is a practical impossibility. Forget it.
Inhibiting detonation is quite enough, and lead is nearly miraculous at that. Many credit the Allied victory in WW II to the use of lead in aviation fuel. Germany and Japan did not have it for reasons not important here, and that was a major disadvantage for them.
100LL has a maximum limit of only two grams per gallon, but in reality, most 100LL has even less. The refineries have learned how to blend a package that produces approximately 97-octane fuel without the lead, and they add barely enough to bring that up to 100 plus a few more points so that they can be sure that their product meets the minimum specification when it is delivered. Lead is by far the most expensive component of the fuel, so they save money by doing it that way. When 100/130 was the standard, the mix limit was four grams of lead per gallon, but in reality the actual blends were around 2.7, for the same reasons. (There are about 6.0 pounds, or about 2,721 grams in a gallon of 100LL, so the lead is less than 0.0735%, by weight.)
There is a crucial distinction to be made here. Please understand that neither lead nor "octane" has much to do with the power content of the fuel! In general, all avgas has pretty much the same BTU content, and all will produce about the same power, provided the timing is adjusted to allow for the faster or slower burn, and provided you avoid detonation by limiting the power. (As a side issue, jet and diesel fuel have more BTUs per gallon (and they weigh a bit more), but are not usable in spark-fired engines. They both depend on introduction of the fuel into the combustion chamber over time.)
"Octane" numbers simply indicate "resistance to detonation." Refineries first raise the octane by blending various mixes. But the more they raise the octane, the more the stuff costs. The current practical limit without tetraethyl lead appears to be about 95 to 97 octane. Basically, if you want more octane than that, you have to use lead to get it.
If the theory of "octane" is simple, the numbers are not. It is a very common mistake in pilot lounges and coffee shops to talk about octane at the gas pump where you fill your automobile, and the pump where you fill your airplane. The calculations are VERY DIFFERENT, and they cannot be directly compared! It's like talking about knots vs. miles per hour, or using "gallons." Is that American gallons, or imperial gallons? There are several entirely different ways of measuring "octane." There is "Research Octane Number" (RON), "Motor Octane Number" (MON), "(R+M)/2" which is nothing more than an average of the two, and "Observed Road Octane Number," (RdON).
Finally, there is the octane number we talk about in GA. It is close to "Motor Octane Number" but not identical. So much for standards. Actually, there are fairly good reasons for several different octane measurements, as "octane" works differently in different situations (race engines, road engines, aircraft engines, air cooled vs. water cooled, intake air temperature, RPM, etc.). For more on this, there's a short but decent explanation at http://www.osbornauto.com/octane.htm.
I'm not much of an environmentalist by modern standards, I guess. I think what the government and the environmental wackos have done to HALON is criminal. But the lead issue stops even me in my tracks, for it is definitely a poison, and known to be harmful. HOW harmful (in avgas), I don't know, but I think it's probably wise to be rid of it in all fuels. Oh, and by the way, if we don't get rid of it, the environmentalists will "help" us do so. I don't want that.
Whether wise or not, like it or not, lead will eventually disappear from avgas, and not just for environmental reasons. It is an expensive solution, and it is becoming ever more difficult to use it. The lead itself is now produced only in Britain and Russia, and only a small handful of refineries produce leaded fuel. The move is on to consolidate that production so that fewer and fewer refineries touch it.
Any tank, truck or pipeline that transports leaded fuel is automatically contaminated with lead, and cannot be used for anything else without a very expensive cleaning process.
There are obvious costs here, and some that are not so obvious. For example, if only one company makes 100LL, then transport costs to distant locations soar. 100LL needs dedicated transport, storage and pipelines, all of which push the price much higher for ALL fuels, not just the leaded fuels.
Now remember, there is NO SUBSTITUTE for lead, we'd better accept the fact that none will be found, and it is going away. What does that mean?
Painting in broad strokes, it appears that about 70% of all GA piston aircraft use about 30% of the current 100LL avgas. These are the low-powered, low-performance aircraft, which can probably run on "100LL without the lead," or on the old 91/96 octane, maybe even on 80/87, and in some cases, mogas. In general, this 70% is not a real problem.
The other 30% of the GA piston aircraft use about 70% of the current supply of 100LL. These are the "high-performance" aircraft, those that fly high and fast, usually with the larger displacements, and often superchargers or turbochargers.
These aircraft MUST currently have 100LL, in order to develop full power. In fact, a couple of the big "flat six" engines are already very marginal on 100LL, sometimes operating in light detonation at cruise power when the CHTs are allowed to get up in the 430°+F range, but still operating as the factory suggests. Will the lack of leaded fuel ground them? Well, maybe not directly, because we COULD run them at reduced power. But how would you like your 300 HP airplane to be limited to 250 HP? Even worse, how would you like to be flying a Navajo, with the big 350 HP TIO-540J2BD engines, and lose one on a hot day, with a limit of only 300 HP on the good one?
The owners of the 70% of the airplanes that burn 30% of the fuel will yell, "Give us a low octane fuel we can use, and find something else for those other guys." Two fuels isn't going to work -- the two-fuel infrastructure is no longer there. Also, if we can settle upon just one fuel for everyone, the price of that fuel will be less, and maybe even less than the lower-octane fuel would have been if produced separately but sold in the much lower volume.
Even if "reduced power" could be used, it would have to be tested and approved by the FAA ... and folks, that just ain't gonna happen. For one thing, it would take decades, and every single model of airplane, with every single possible combination of powerplant would have to be run through a test program and recertified. Further, the practical reality is that just about every airplane out there could use more HP, not less. We are already limited in range when carrying a load, or limited in load when going any distance. It's simple physics.
So the practical reality is that if 100LL went away tomorrow, those aircraft now using about 70% of all avgas would be grounded forever. That's simply not acceptable.
Alcohol? Forget it, it has very significant problems of its own (certification, for one), and because it offers less BTU per gallon, it cuts range by about 15%. It has been tried, and tried hard. It's a non-starter.
I've heard some folks suggest that FADEC ("Full Authority Digital Engine Control") is somehow a solution. Folks, even if it works as advertised (it doesn't, and won't) FADEC has nothing to do with the fuels issue, it is merely a crutch for those who whine they can't be bothered with understanding props, throttles and mixtures, and how badly we need a single control like a jet engine. I'm all for simplification when it makes sense, but FADEC in its currently advertised configuration is worthless. The current FADEC effort by TCM's Aerosance group is not only worthless, but, based on present early reports from those that have tried to use it in the field, I predict a giant failure. (More in a future column, I'd like to fly it myself before saying much more.)
There is a most elegant and UNIVERSAL solution to this whole mess, and it's called PRISM. That stands for "Pressure Reactive Intelligent Spark Management," and it's running, right now. It looks directly at the combustion event, and alters the timing of the spark to optimize it.
Variable timing is not new. The old R-3350 engines on the Connies and DC-7s had it, but it was only a two-position switch, one selected "Cruise," the other was for "everything else." Worked great. It allowed the timing to be set closer to optimum for the high power settings, then selected to a "better" timing for cruise, thus saving fuel.
Modern autos use variable timing, but in ways that are not suitable for aircraft. They measure a large variety of parameters and take a guess at what ignition timing and fuel flow might work best for EMISSIONS, not economy or power. The feedback for fuel flow is "closed" around the oxygen sensor (i.e., the rough equivalent of peak EGT) and that is clearly not optimal for aircraft engine operation. The spark timing is controlled by continually forcing the timing up against the knock sensor, and that, for sure, is not what we want to do in an aircraft engine!
No one has had much luck in getting traditional automotive knock sensors to work in our aircraft engines, because there is far too much extraneous noise. Unison tried that with their LASAR system, and gave up -- the normal noise of the aircraft engine buried any chance of hearing a knock from detonation.
In short, the automotive systems are bastard compromises employed to meet environmental issues and cost limitations on the sensors used in automotive mass production.
It took the incredible modern electronics now available and lots of hard work and long hours by some very, very sharp people at GAMI to truly control timing in a remarkably simple way. Instead of a large number of parameters and a very complex computer mapping array, PRISM looks at only two, the position of the crankshaft, and the actual pressure in the combustion chamber in real time, at very high speed (50,000 times per second ... for each cylinder).
I wish I owned stock in GAMI, because if they can get this system past FAA certification, I absolutely believe it will revolutionize the gasoline-powered, spark-fired, internal combustion engine, whether it's in your lawnmower, your car, your airplane, or an R-2800 radial.
Snake oil? Pie in the sky? NO. I've SEEN it run. I've run it myself. It works. More about it in a future column, we're talking about fuels, here.
By far the best solution is for the petro companies to simply stop adding lead to 100LL, and produce what remains, or something very similar to what remains, as THE single aviation fuel. GAMI has obtained a batch of "unleaded" 100LL from some friendly folks in the industry, and have run a TIO-540J2BD Chieftain engine on that fuel. More on this below.
Reports are that one of the Chevron refineries makes 100LL on a routine basis that, with the lead package removed would test out at 95-97 (motor) octane or better. One would think that if Chevron can do that on a routine basis, then it would not be too hard for all the oil companies to produce this mix with a minimum standard of 95 octane. With this fuel, we have a future… even a bright future.
Apparently BP claims to have a 100-octane unleaded fuel they are working on as a proprietary product. That would be nice. The more choices we have, the better!
But, for now, lets give this future 95-octane fuel a name: "95NL" for "no lead."
Pay attention, here! YOU, in the back, wake up, or get out! Listen up, people!
The most detonation-critical engine in all the GA fleet -- the Lycoming TIO-540J2BD engine used on the Piper Chieftain -- can meet FAA certification requirements, and produce its full rated 350 HP on 92.5-octane UNLEADED avgas, using PRISM.
What will it do on 95NL? Well, it will either run better with better detonation margins, or it can be made to produce more power, of course!
Notice I'm using the present tense, not the future? The Lycoming TIO-540J2BD engine IS running RIGHT NOW on the world's most sophisticated test stand at GAMI's Carl Goulet Memorial Engine Test Facility in Ada, Oklahoma.
NOTE: Carl Goulet retired in 1993 as the V.P. of Engineering at TCM. After retirement, he worked as a Designated Engineering Representitive with the folks at GAMI and supported their certification efforts for the development of the GAMIjector fuel injectors. He was one of the few who truly understood the engineering. Once he retired, and the engine companies fired or let go a number of others with lots of experience, there's was no one left. The last 40 years have not been an inspiring period in the history of GA piston engines.
Many "less-difficult" engines than the TIO-540J2BD will probably operate just fine, producing full rated power on 95NL, even without PRISM technology. With PRISM, they will be capable of producing MORE power, and at significantly more efficient BSFC numbers.
By using the well-known 100LL-base product and just removing the lead, the compatibility problems disappear. The vapor pressure would remain the same. There is no need to modify any plumbing. Current O-rings, seals and fuel tank bladders are just fine. Refineries will not need to change a thing, just "leave the lead out," and they can transport our fuel on any truck, or through any pipeline.
There are some certification issues if you start talking about pulling more than the HP for which an AIRFRAME is currently certificated. While this can be done, it's not easy, and few will bother. However, with PRISM, that full rated power will be available at the higher altitudes, and higher cruise powers will be available, as well, with better fuel specifics.
With 95NL avgas, most of the GA fleet of piston-powered aircraft can continue to fly, with at least the same performance, without modification. For the few that won't, PRISM will allow that. PRISM will improve the performance on ALL engines, including my beloved big radials, and reduce the adverse effects of peak combustion pressures that play a significant role in engine durability.
Here are some realities, as I see them:
- If we end up with a 91 or 92 octane unleaded fuel, we can, with PRISM or other technology that may appear on the horizon, most likely continue to run our engines.
- However, if we end up stuck with a 91 or 92 octane fuel, then there is no known means to be able to continue to make improvements in our engines.
- If we end up with a good 95NL, then not only can the fleet continue to operate by using new technology like PRISM, but there would continue to be some "headroom" to make many of our engines run better in the future than they run now on 100LL.
What we really need is for everyone to start yelling for "95 No Lead" as THE single aviation fuel. Stop talking up alcohol. Stop thinking there's some magic non-toxic additive, because it probably doesn't exist. With 95NL and some further work on the certification of the currently known and demonstrated technology, the problems can go away, and we can "rock and roll."
Be careful up there!
John Deakin is a 35,000-hour pilot who worked his way up the aviation food chain via charter, corporate, and cargo flying; spent five years in Southeast Asia with Air America; 33 years with Japan Airlines, mostly as a 747 captain; and is now flying Gulfstream IVs as a contract pilot. He also flies his own V35 Bonanza (N1BE) and is very active in the warbird and vintage aircraft scene, flying the C-46, C-131, DC-3, F8F Bearcat, Constellation, B-29, and others. He was also a National Designated Pilot Examiner (NDPER), able to give type ratings and check rides on 43 different "vintage" aircraft, until the powers-that-be took a dislike to his writing style, here and elsewhere.