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 past 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.
Actually, the octane rating is actually a combustion factor. Octane is actually a ignition suppression element and is why different engines are built to run on it or not.
I’ll wait for the real-life test results from the AOPA Beech Barron, both engines overhauled. HEAD-TO-HEAD. now one engine low lead only, the other no lead only. Mike Busch at Savvy Aviation is not skeptical of lead-free at all, and his shop has developed a special valve inspection & measurement to get to the bottom of this issue. Data! We need data.
How about replacing the exhaust valves with sodium filled or using titanium valves on your next TBO? Is that even an option? I understand that it’s not a quick or easy process to manufacture, test, and certify anything with aircraft, but if there’s not much time left with LL, shouldn’t people be working on something to keep those older engines working into the future? I’m just shooting off the cuff and not claiming to know all that much about aircraft engines.
Air cooled engines have enough problems without deleting the lead needed for surface lub – cooling – elevation temperature – ice – freezing rain – hot desert wind – come on man get it right ✅️ nutons law did.
or, maybe newton’s laws….
…whatever they have to do with engines ?
Please explain to the ‘unwashed’ why
NON ETHANOL MO GAS of 87-91 octane isn’t useful ?
Thanx,Unwashed One
An excellent book that covers engine tech, octane rating (test eng vs real world) among others is “The Secret Horsepower Race/Western Front Fighter Engine Development” by Calum E Douglas. He’s a former F1 engineer who dug into archives of when this was all leading edge research. Gives an appreciation for challenges then/now with fuel formulation vs eng design vs application. Not a casual read and spendy, but rare combination of tech and historical depth.
I’m mystified as to the search for unleaded fuel. We’ve had the ability to use unleaded automotive fuels in light aircraft for decades. Just getting more non-ethanol mogas manufactured would have been much simpler. Seemed like an expensive solution to a solved problem.
Ed, Mogas only would work for part of the fleet, not everyone.
the unsolved problem — airports (except in the high tier States) won’t dispense Mogas in their Fuel Farms, so we have to truck it in in 5 gallons cans from off-base.
Is the “orphan” population holding up the masses that are already approved by their manufacturer?
Seems the automotive world adjusted and those “orphaned” had their valves changed if the cowners wanted to keep operating them. Although I have a number of lead fuel engines that have yet to have a problem with unleaded.
On the other hand a vintage vehicle can get the conversation for hundreds of dollars, whereas an aircraft will be several thousand. Perhaps that’s the real problem.
A car, truck or tractor can change engine parts at will and at very little expense. An aircraft cannot change even a small engine detail without going through an expensive FAA certification process. This does not cost thousands, but millions of dollars to accomplish, and the cost must be absorbed by a relatively small number of aircraft when compared to the number of road vehicles. That is why what is economically practical on a road vehicle is prohibitively expensive on an aircraft .
“Most of the trucks switched to diesel and… ” Bingo. There is the simple answer to future aircraft engines, Diesel-cycle designs burning Diesel and/or Jet-A. Trucks and Jet-A aircraft are the most ubiquitous form of transportation in the world. I suppose one could also count in diesel/electric locomotives and ships, which move nearly all the freight in the world. Diesel is going to be around for a long, long time. Let’s Go Diesel!
It was bad enough when 80/87 was phased out, and our low power engines had to drink the green stuff. Enough engine problems resulted from that.
It’s too bad that so much time, effort and trainloads of money has been spent trying to develop UL avgas, when avgas consumption is a miniscule amount compared to gasoline of all types. It seems in vogue to jump when a tree hugger cries!
Lead isn’t just an environmental problem, it also has the issue that it is objectively awful for engines in every way other than those mentioned above (antiknock and valve seat wear). It fouls spark plugs, leaves nasty deposits on the valve stems causing them to stick, pollutes the engine oil with lead or lead oxide particles, etc. Furthermore the fuel has to contain lead scavengers to prevent build up of combustion chamber deposits, those lead scavengers are usually chloro- or bromo- alkanes that when burned produce hydrochloric and or hydrobromic acid, which further contaminate the oil and corrode internal components. Basically if you can safely switch to unleaded your TBO will improve by a lot.
Nice to see a story about the valve recession problem which we probably need to address now to cut off future problems with the removable of lead, which seems to help with heat transfer at the exhaust valve. It would be nice to see the FAA take some leadership here in requiring new hardened valve seats to minimize the problems in the future.