When I was working for Shell, I had a sign hanging on my office wall that quoted Murphy’s Law: “Anything that can go wrong will go wrong.”
Under it I hung another sign that said “Murphy was an optimist.”
This brings me to a much-asked question about the unleaded replacement for 100LL: What could possibly go wrong?
There are many things that could go wrong in just the supply and production of the product.
But from a technical aspect, I think that there are three main concerns:
- Knocking
- Component incompatibility
- Exhaust valve recession
Many people think that as long as the new unleaded fuel meets the 100 lean rating specification (spec) it will have equal performance in the field as 100LL.
The problem here is that an octane rating is not a physical property of a fuel. It is a number based on how well the fuel did in a test engine. And a fuel may not perform the same in a single cylinder test engine at 900 rpm as it does in a 2000 cubic inch super charged radial at 2800 rpm.
For example, when 100/130 fuel was replaced by 100/130LL, there where numerous knocking complaints, especially from the radial engine people. Many of them had to reduce the maximum boost pressure allowed on takeoff to eliminate the knocking.
This, in turn, reduced the useful load allowed and the profitability of the aircraft.
The other problem is how does one determine when an aircraft engine is knocking before it does any engine damage?
If it is a big radial engine mounted out on a wing, it is making so much noise and vibration that a pilot cannot usually tell if it is knocking or not.
The people at General Aviation Modifications Inc. (GAMI), which developed G100UL, an unleaded alternative to 100LL, have done an excellent job of studying the knock characteristics of their product in flat engines. We will have to see how that translates to large radials and others.
GAMI officials note that when the company received its approval from the FAA for its unleaded fuel, the Approved Model List covers “every spark ignition piston engine and every airframe using a spark ignition piston engine in the FAA’s Type Certificate database,” including all of the World War II engines and all of the post-World War II radial engines.
Then there is component compatibility problem.
This is one of the reasons that ASTM has not been able to develop a new specification for unleaded fuel.
The present ASTM D 910 spec is written around fuels that are a blend of aviation alkylate and some toluene concentrate, the lead in low lead.
Writing a new spec that covers any and all possible fuel candidates to be used in every old and new aircraft ever produced is an almost impossible task.
I have written about exhaust valve recession before (What causes exhaust valve recession in an engine?).
In a four-cycle spark ignition engine, you have the suck, squeeze, bang, and blow cycles. Following the bang or power stroke, the exhaust valve is opened and exhaust gases that are more than 1,000°F are forced pass the open valve. This heats up the valve edge significantly.
To cool that area, heat is transferred up the stem to the guide and the seat when the valve is closed.
The lead by-products of combustion tend to coat the seat area and improve the heat transfer.
When the lead is removed, the heat transfer rate is reduced and the seat temperature is elevated, which increases the erosion rate of the seat.
When the automotive world went to unleaded fuels in the early 1970s, exhaust valve recession was a problem, especially in gasoline-powered trucks. To solve the problem, manufacturers installed hardened exhaust valve seats in cars and pick-up trucks. Most of the trucks switched to diesel and the few farm tractors that still used gasoline were not affected too much because of their lower operating rpms.
Aircraft engines operate at higher rpms, high load factors, and they are air cooled, so the seats run hotter.
That means recession could be a problem when flying with unleaded fuels.
Years ago, Lycoming started using hardened exhaust valve seats in all of its engines. Now I understand that Continental is also using hardened exhaust valve seats.
My question is: Who is going to approve and make sure all of the orphan engines that are being rebuilt are also using hardened exhaust valve seats in their engines?
This would be a good project for the folks involved in the Eliminate Aviation Gasoline Lead Emissions (EAGLE) initiative to take up.
Failure to do that could lead to significant problems and have a negative effect on general aviation as a whole.
