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.
April 27, 2002
|About the Author ...
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
now flies the Gulfstream IV for a West Coast operator.
He also flies his own
V35 Bonanza (N1BE) and is very active in the warbird and vintage aircraft
scene, flying the C-46, M-404, DC-3, F8F Bearcat, Constellation, B-29, and
others. He is also a National Designated Pilot Examiner (NDPER), able to give
type ratings and check rides on 43 different aircraft types.
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...
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.
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 "Pelican's
Perch #43: Detonation Myths.")
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
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.
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
"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.
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
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.
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
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.)
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
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
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
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
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,
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
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
Be careful up there!