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Avgas replacement: chicken, meet egg: GAMI'S new G100UL may be a drop-in replacement for 100 LL. the challenge will be devising a test program to fast track it and get refiners interested.

Compared to piston aviation fuels research, mushroom farming is a daylight operation. That's not to say the fuel work is secretive, it's just that it goes on more or less constantly, but nothing meaningful seems to come of it. At least you can have the mushrooms on your salad.

Against this backdrop of apparent non-action comes yet another entrant into the 100LL replacement sweepstakes, this one called G100UL. This new fuel comes at the problem from far out in left field from a company known more for burning fuel than creating it: General Aviation Modifications, Inc., the Ada, Oklahoma, mod house that shook up the hidebound world of aircraft engine research with its radical ideas on lean-of-peak operation and an almost religious conviction that turbo-normalized engines are better than turbo-charged engines.


With G100UL, GAM I is again running against the grain and, to a degree, challenging the accepted notion that before a new fuel can be widely tested, it has to be certified. But, says GAMI's George Braly, that's backwards. There's no point in reducing entire forests to pulp to certify a fuel if refineries aren't interested in or can't build the stuff profitably, thus GAMI's idea is to field its new developmental fuel to a select fleet under an STC while simultaneously pursuing regulatory approval. To a degree, that will test the economics, too, since production will have to rise to at least the pilot-plant level to supply a small fleet experiment.


The very existence of 100LL has been under threat since amendments to the Clean Air Act of 1970 eliminated the use of tetraethyl lead as an octane enhancer in motor fuels. Because it couldn't be shown that aircraft engines could safely operate on lower octane unleaded fuels, general aviation has operated under a "temporary" lead-content waiver for the past quarter century.

Year after year, the government has been talked out of or just hasn't shown committed interest to drop the hammer on TEL once and for all. However, the current urgency to find a replacement for TEL's octane pop comes from a new lead initiative by the EPA.

A petition filed by the Friends of the Earth in 2007 asked the EPA to set new, more stringent standards for lead air pollution. As part of that regulation, EPA is requiring air monitoring around airports to determine what levels of airborne lead exist, but the new rule sets 2011 as a deadline for reducing lead emissions. (See sidebar.)

It's impossible to predict where this monitoring will lead or how fast it may develop into a regulation that would eliminate 100LL entirely. Braly thinks the danger is clear and present and has thus launched his own fuels research project on an engineer's hunch that the solution may reside in current technology.

In fairness, it's not like the industry hasn't been trying. Between the engine manufacturers, the FAA's Atlantic City William J. Hughes Technical Center and the oil companies, lead-free experimental blends in the dozens have been tried.

More than a few have proven promising, although none have the attractive combination of TEL's seamless octane enhancement and relatively cheap price. An insistence that a lead-free fuel exactly meet the avgas standard, ASTM D910, has complicated the search for a replacement by eliminating some near-hit choices. Braly believes G100UL meets D910 with no significant departures.



GAMI invited us to the company's Ada, Oklahoma, research facility to observe some test cell runs and actually fly with G100UL. So what is the stuff? Braly declined to reveal the exact formulation due to proprietary and competitive concerns, but says he will reveal more when further testing is completed.

Braly did confirm that G100UL is largely high-octane, unleaded gasoline feedstock blended with components that are more or less standard hydrocarbon chains from materials that any refinery can derive from known petroleum refining techniques. It's not bio-derived, as Swift Fuel is.

However, in the recent past, some fuel formulations have been proved to work including ethyl tertiary-butyl ether (ETBE) or methyl tertiary-butyl ether (MTBE). Although these compounds are out of favor for being implicated in groundwater contamination, research indicates they do provide the required octane. In Europe, Hjelmco Oil has developed an ETBE-laced 100-octane fuel that the company's Lars Hjelmberg says could be certified in the U.S. The FAA has no beefs with ETBE, but EPA might.

Another possibility tried before is the so-called high aromatics, such as toluene or the benzene groups. This is essentially how Swift Fuel is blended, using bio-derived acetone as a feedstock. It seems plausible that the same thing could be done using petroleum-derived feedstock by simply manipulating the distillation process, which is what refining is all about anyway.

"Yes, it might work, I suppose," says Rob Midgley, Shell's global technology manager, when we asked about the plausibility of a Swift Fuel approach derived from oil instead of saw grass. And what of the economics?

"Oh, who knows?" he said, which mirrors the reaction we get from other oil refining insiders when asked how plausible the numbers are. The problem is that refineries, despite buying billions of dollars worth of crude a year, operate on tiny margins. A misplaced decimal point in a feasibility study can turn what looks like an acceptable profit into a disaster.

Braly concedes that this is a major hurdle that G100UL will have to leap. "This has always been a chicken and egg problem," he told us. "If you're going to get the refiners interested, you have to show them a fuel that works, but if you're going to show them it works, you have to build enough of it to test it widely."


Toward that goal, by early February, GAMI had run about 25 hours of test cell work on a beaten down IO-550 used for its Cirrus turbo work. In addition, it has flown three test flights, one of which we came along on to help collect data and observe results.

The single largest concern--apart from economics--is motor octane rating. The ASTM D910 spec for avgas calls for 99.6 MON minimum, but most FBO 100LL is a bit over that, at 101 to 103 octane. Presumably, G100UL would have similar variability due to vagaries in the refining process, but Braly told us in the laboratory test version, the fuel easily exceeds the D910 minimum.

At that octane, you'd expect G100UL to have good detonation characteristics and it does. In the test cell runs we observed, 100LL and G100UL had detonation profiles that were too close to distinguish and both have more than enough margin to meet typical FAA detonation standards.

Braly also ran some 95-octane unleaded fuel and some so-called "min spec" avgas, whose octane was actually at the lowest end of the ASTM requirement. Both of these induced light to medium detonation at cylinder head temperatures not much above what most of us would consider the high side of normal. A couple of octane points make a dramatic difference in delaying the onset of the detonation.


ASTM D910 has a number of other requirements including vapor pressure, boiling and freezing points, sulfur content, corrosion characteristics, aging and oxidation requirements and water reaction qualities. A significant worry about fuels with high aromatic content is seal, hose and O-ring swelling.

"There have been problems with aromatic contents higher than about 40 percent in aviation fuels. It's not just as simple as raising the octane number," Shell's Midgley told us. So-called phase separation of water has been an issue in fuels with ethanol but it appears not to be in G100UL. In fact, Braly told us G100UL meets the D910 spec line for line with the exception of having no lead, slightly lower energy per unit of weight and some departures on distillation end points. As for seals and hoses, soak testing was underway when we visited, including containment in a compos ite tank. Braly said no problems had cropped up.

The fuel's energy density produced some interesting results in the test cell and in the airplane. At equivalent power output, in one case about 319 HP, we noted that fuel flow for G100UL was 26.7 GPH but 28.2 GPH for 100LL.

But because G100UL is heavier, there's a payload hit--it would be about 30 pounds for a 75-gallon fill-up. But you can get the same range for less fuel volume, so the overall impact on range is under 5 percent. With full tanks, range is extended by about the same amount. In other words, G100UL is almost a wash against avgas. You'd hardly need to factor it into flight planning. Swift Fuel is considerably heavier at 7 pounds per gallon. The same 75-gallon tank would be 75 pounds heavier and although Swift also has a higher energy density--13 percent--the volume-against-weight equation means slightly less range. In our view, this isn't a significant operational consideration, either.


The FAA doesn't quite know what to do with G100UL, because it's being developed in a sort of business-to-business model and not the historic FAA/Coordinated Research Council network. Braly has proposed to the FAA that G100UL be fast tracked by approving an STC to test it in a limited number of Cirrus aircraft. He argues that these airplanes are equipped with sophisticated engine monitoring, providing no better opportunity for accelerated fleet testing while GAMI pursues ASTM approval.

However, the FAA is pushing back on this idea, preferring to leave all fuel approvals to the ASTM process, which could take months, if not years. Meanwhile, the EPA's lead-regulation clock appears to be ticking.

At this point, it's too soon to declare G100UL the winner of the new aviation fuel sweepstakes. We don't know enough about its formulation and economics. But given that it's petroleum derived rather than bio-mass derived, it looks for promising for now. The to its future is how fast a testing and refinery project can move forward to prove it out. Or not.


Setting aside lesser considerations like energy density and distillation points, the Holy Grail of aviation gasoline is octane and the graphs on this page tell the story. For low-power engines, lower octanes-about 95 MON--will suffice. But larger displacement, high-power and turbocharged engines require higher octane because higher cylinder pressures and temperatures reduce detonation margins. Higher octane fuels, because they burn with a slower, more consistent flame front, increase detonation margins.

GAMI's sophisticated test cell is configured to run detonation tests by using sensitive pressure transducers that sample cylinder pressures many times a second and analyze patterns in near real time. Detonation is detected as the onset of pressure spikes that indicate the flame front is burning explosively. So-called light detonation isn't much of a concern, but medium and heavy detonation can damage rings, pistons and valves.

The labeled graph at upper left shows three key test cell parameters--peak cylinder pressure, CHT and piston top dead center. The test cell's software is configured to sample pressure and plot the onset of detonation as a dot on the TDC line whose color corresponds to intensity. (Yellow is light detonation.)


During our demo, we were shown detonation profiles on several fuels, including 95UL, min-spec avgas, FBO 100LL and G100UL Because of space limitations, we've chosen to show a detonation comparison between G100UL and 95UL The left graph below shows the onset of detonation in ail six cylinders of the 10-550 test article, with high power set (116 percent) and CHTs creeping above 400 degrees, but below the published redline limits for the engine.


The graph on the right shows the same engine running on G100UL at 111 percent power with the hottest CHT at 405 degrees F. During a series of runs, we saw higher CHTs than this, without serious detonation.

Although we don't have room for the graph, the 100LL detonation profile was all but indistinguishable from the G100UL graph, indicating to us that its detonation margin is close enough to be called the same.

One other parameter not shown here is so-called ThetaPP, the difference in angular crankshaft rotation at which peak cylinder pressure occurs after TDC. A larger angle indicates a later pressure spike which is, ail things considered, desirable. The tests indicate G100UL has a slightly larger ThetaPP, something that octane alone might not necessarily indicate.



For at least three decades, the Clean Air Act's prohibition against lead in fuels has been somewhat toothless. Year after year, amendment after amendment, the Environmental Protection Agency has given leaded fuel for aviation a pass on further regulation.

It has always been assumed that the complete banning of lead was an inevitability and a consortium of environmental groups called Friends of the Earth is spending dollars to make that happen, in 2006, FOE petitioned the EPA to examine the environmental impact of leaded aviation fuels and later asked for more stringent regulation.

As part of that study and a subsequent tightening of lead air pollution standards, the agency began looking at emissions and contamination around airports, with some controversial sampling at the Santa Monica Airport in California.

The EPA has identified avgas as "a significant source of lead," but AOPA has argued GA airplanes produce only .1 percent of all lead emissions in the U.S. The latest development that raises worries about the EPA's seriousness is that it has ordered new air monitoring at airports to support its findings from the Santa Monica study. In all, 55 airports have been named as potentially exceeding proposed lead content thresholds.

This sounds ominous, but is it really? Yes and no. AOPA's Rob Hackman told us the association is monitoring the new EPA initiative, which it sees as cause for concern, but not alarm. Yet.

"Does it mean the immediate death knell of 100LL? Probably not," says Hackman. For one thing, the 55 airport list will probably be winnowed down and this is not yet a full-blown federal assault. The EPA is leaving the air testing up to state environmental agencies, so local airports shouldn't bear the cost burden.

GAMI's George Braly says EPA's presentation at AOPA Summit last fall "scared me to death." He believes GA could get caught flat footed, which is why is launched the G100UL project.



(+) Test cell runs indicate that G100UL has detonation margins equal to 100LL

(+) The fuel is a little heavier than 100LL but has higher per-volume energy density, thus it's nearly a wash with 100LL

(-) Test project came out of nowhere and the FAA is resisting fast track development.
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Title Annotation:FUTURE FUELS
Author:Bertorelli, Paul
Publication:The Aviation Consumer
Geographic Code:1USA
Date:Mar 1, 2010
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