For many years airplanes have been the only transportation vehicles still allowed to use leaded fuel in the United States. Even NASCAR transitioned to unleaded gasoline for its stock car races a couple years ago in a symbolic move of going green. In aviation we have had a solid defense for leaded fuel because there has been no other option. Many high-powered piston engines can’t operate to their full power potential without the knock retarding presence of tetraethyl lead in the fuel. Aviation did its part in the three-decade-old effort to ban lead from motor fuels by creating 100 low lead octane avgas, which contains a fraction of the lead used in the original 100 octane formula. But there is still lead in 100LL and even the comparatively tiny amount of lead exhausted by all piston airplanes is too much for society to accept indefinitely.
There are several unleaded gasoline formulas that can power the majority of aircraft piston engines because the biggest number of engines are the low-powered naturally aspirated type that power the huge fleets of Skyhawks, Cherokees and other light singles. However, the actual use of avgas is stood on its head by the fact that the majority of the fuel is burned by a minority of high-powered airplanes that fly more and consume more fuel than the lighter airplanes. Since FBOs are realistically able to supply only one version of avgas, a 100LL replacement needs to serve the entire piston fleet, or at least come as close as possible to that goal.
As far as I can learn lead began to be used in aviation fuel as early as World War I. At that time the design of piston engines and the manufacturing of fuel for them were both in their infancy and the results were uneven, to say the least. An engine, under the stress of wartime flying, may perform fine on one batch of fuel, but then self-destruct quickly on another tank of fuel. The term gasoline was not universally used at that time and the actual chemicals in the fuel varied widely.
Eventually engineers figured out that it was detonation that was literally tearing apart the piston engines of early fighters. Under normal operation the fuel-air charge in a piston engine burns quickly, but it burns. The problem occurs when the charge detonates, in other words it explodes instead of burning. The explosive force of detonation can be heard in a large iron block engine such as found in cars as a knock, or a ping. Detonation also damaged car engines before it was well understood, but with their greater mass it was a longer term process of destruction. In a lightweight aircraft engine detonation could break or melt the crucial components in a matter of seconds.
Experiments found that lead in the fuel would prevent detonation, or at least keep it at bay until extreme pressures and temperatures were encountered. I’m no chemist so I can’t explain exactly why lead works this magic, but it does.
To determine just how resistant a fuel is to detonation, calibrated test engines were developed that could adjust temperature, rpm and other factors to determine at what point a specific fuel formula would detonate. From those test engines came the now familiar octane rating. The engineers who developed an engine determined the conditions it would encounter and specified a minimum octane fuel that would prevent detonation. In general, the higher the engine operating temperature, air temperature, rpm and compression ratio the higher the octane rating must be to avoid detonation.
There are at least three octane rating systems in general use. In aviation we use the motor octane number (MON) rating, which typically represents the harshest operating environment for a piston engine. The other rating method is called research octane number (RON) with knock resistance measured under less stress. To add to the confusion, in the U.S. automotive gasoline is rated using the pump octane number (PON) that is an average of the other two test results. That means you can’t compare the knock resistance of autogas of a reported octane directly to that of avgas.
But an even bigger issue than different octane rating systems when it comes to using auto gas in an airplane engine is the variability of auto gas. It’s not that auto gas is an impure fuel, it’s the fact that auto gas is not made to a uniform specification. Oil companies adjust characteristics of auto gas for the season of the year and various climates across the country. For example, in the Northern tier of the country auto fuel is designed for easy cold starting in the cold winter months. When summer comes, the blend will be adjusted for hot weather operation. In the high-elevation parts of the West there is still another blend, while warm Southern states get fuel to suit their driving conditions. On top of that, the EPA requires specific additives such as ethanol in some areas, but not others, and not always year around.
While the auto gas supply chain works beautifully for our cars, it is not the answer as a 100LL replacement. Yes, there are STCs that permit use of auto gas in most lower-powered airplanes where detonation margins are wide. But even those STCs are problematic because most did not consider the effect of ethanol, which is increasingly common, particularly in the blend sold during the warm months. Ethanol may provide sufficient detonation margin, but it is not certain that it won’t attack and dissolve materials used in airplane fuel tanks, fuel lines and fuel injectors or carburetors. The recreational marine industry — particularly outboard motors — has had very serious problems with ethanol and we should expect some of the same problems in airplanes.
For a number of years the people at Teledyne Continental have believed that it is possible to develop an unleaded avgas formula that will have sufficient detonation resistance for use in the huge majority of piston engines. Over the past year Continental has become very serious in testing such a fuel and has demonstrated that its most popular engine versions can operate on unleaded fuel in the test stand, and now in flight. The results are very encouraging.
Continental’s fuel of choice is 94UL (unleaded) avgas, which is actually 100LL without the lead. The missing lead costs six octane points, which is not insignificant in high-power engines. However, because the 94UL has all the same properties as 100LL there are no unanswered questions about vapor lock, long storage life, or compatibility with aircraft fuel tanks and fuel systems. Refiners around the world know how to make it because the process is the same as 100LL, except the lead is not added at the end.
Continental’s testing so far has shown that even its big bore 550-cubic-inch engines can operate just fine on 94UL. After proving it on the test stand, Continental flew an instrumented 550-powered Bonanza on 94UL including climbs and maximum cruise with no problems. So far it looks like 94UL could become an approved fuel for nearly all piston engines with little or no modifications required to the engines.
However, there are the specialty applications where it remains to be demonstrated that 94UL can replace 100LL. Among the unknowns are the large radial engines that still power many of the firefighting water bombers used every fire season. Most of these engines trace their roots back to the 130 octane avgas days so 100LL is already a step down in detonation margin. The engines will, of course, run on 94UL, but may have to have power output limited by lowering turbo boost, for example. Less power translates into lower payloads so less water to the fire.
Some other hot running turbocharged engines may also have issues with 94UL. I don’t see the geared 520 engines used on the Cessna 421 and a few other airplanes on the list of engines that Continental has tested with 94UL yet. There may not be a problem, but engines such as those, and the Lycoming 541 on the Beech Duke, are making lots of power, and are thus subject to high stress and the threat of detonation.
One solution to engines that need more than 94UL is the use of fadec computer control. By manipulating spark timing and fuel delivery the fadec could prevent detonation just as the computers do in all modern automobiles. Continental has been working on fadec for piston engines for years, and has several FAA approvals, so they do know how to do it.
I, for one, believe that the amount of lead emitted into the atmosphere by piston-powered aircraft is so tiny that it just doesn’t matter. But it also doesn’t matter what I think. Lead in all forms and in any amounts is at or near the top of every environmentalist’s hit list and being the last activity to burn leaded fuel puts aviation in their bulls-eye. We’re talking when, not if, for the end of 100LL.
But Continental’s test results have given me more confidence of availability of a usable 100LL replacement than I have ever had before. It looks like both the engine makers and the aviation fuel refiners could react quickly to make the transition. And life can only become easier for the avgas transportation system and FBOs once the lead is gone. And leaving the lead out can only reduce the high cost of avgas, at least a little. It looks like we can finally get there from here.