Leaded Aviation Fuel?

[Brian121804]

Who'd a thunk ya could learn so much about airplanes on a boating forum? :wink:

 

[C&K]

Who'd a thunk ya could learn so much about airplanes on a boating forum? :wink:

The marine and small engine industry has given ethanol blended fuels a bad rap with lots of myths, despite the fact it is used in millions of automotive engines every day without a single problem. I use E15 in my 80's outboards and not a single problem with it. But I've rejetted the carbs for it.

So it's only natural that people look for alternatives to ethanol blended fuel, even though non-ethanol blends are more expensive and getting harder to find. But leaded avgas is not a viable alternative. It's kind of like asbestos, once used in building insulation. It will go the way of the dinosaurs and become extinct because leaded avgas is not, and never was, a good motor fuel. It is a relic from the 1940's that causes deposits, damage and premature failure to any engine it is used in. The aviation industry just needs to get off its collect butt and shoulder the cost of more modern engines and fuels. And quit complaining about the cost because they are not cheap, or even practical, in the first place. GA piston airplanes are nothing but expensive toys that only a very small portion of the population can afford. So I expect that at the present rate of phase-out, within 3 years you will not be able to 100LL anymore. And for the one refiner that will still refine it (Shell), and the few airports that will still carry it, the price will go $10/gal, or more.

 

[poorthang]

I'm going to admit the octane discussion is way over my head. But, I do have a question. If the leaded octane fuel is such a big maintenance issue, why does aviation gaso have so much lead in it?

In aircraft engines the valve seats and valve material can't be changed without getting a STC (Supplemental Type Certificate), which is very expensive to get. So the same materials are used for replacement parts at overhaul as what the engine was originally certified with. Those certifications (called a Type Certificate) were obtained back in the 40's and 50's when the engine was originally designed and certified.

So, unfortunately, they still require the lead for valve seat lubrication, and these are air-cooled engines that run cylinder head temps of 435-450F at full power, 375 at normal cruise power. Lubricating oils formulated for automotive engines can't withstand those kinds of temperatures, and neither can automotive fuels - vapor pressure is too high with automotive fuels. There are STC's you can get for some engines (like the Continental O-200) to allow use of automotive gasoline. But the STC limits the altitude the aircraft can be operated at, and shortens the TBO (Time Between Overhaul).

The general aviation industry is badly in need of more modern engine designs that can use unleaded fuels. But it is too expensive to get the certifications for it, and the general aviation market can't bear the cost. Commercial aviation switched to turbine power long ago. But general aviation can't bear the cost of turboshaft engines either - a typical small Pratt&Whitney Canada PT6 engine costs $1.1 million, where the piston equivalent costs ~$80,000.

Running 100LL in your outboard at the cold temperatures that outboard engines run at will only cause extreme lead/carbon buildup on spark plugs, piston crown, combustion chamber head and exhaust ports. It will build up in the top ring land and seize the ring, resulting in a scored cylinder.

Unfortunately, this also happens to aircraft engines. Take like a Seneca V with twin turbocharged Continental TSIO-360's, the TBO is 1,800 hrs. But the engines will have to be "topped" at around 1,000 hrs, meaning the cylinders and pistons are replaced with new to make it to TBO, without removing the engines from the airframe. The cost to "top" a pair of TSIO-360's is around $37,000, and major overhaul is ~$57,000 apiece. The cost of the airplane is $1.15 million new for a 2022 model. This is just a light personal twin. If they replaced those two piston engines with a couple small PT6 turbines, the cost of the airplane would be over $3 million. Turbine powered GA personal airplanes are very expensive. A single-engine TBM940 is a $4.5 million dollar airplane and it costs $744/hr to stick the key in the switch and start it.
https://air.one/aircraft-showroom/daher-tbm-940

This is an accepted part of the cost of flying airplanes - nothing about them is cheap. It's not an accepted part of operating outboard motors.

Some people still do run avgas in certain 1960's muscle car engines. But if you put that engine on a dyno and compare 100LL avgas to Sunoco Optima 95 race gas, you'll find you get way more power with the race gas. 100LL avgas is 112,387 BTU/gallon (18,700 BTU/lb). The old MTBE formulated automotive gasoline was 124,880 BTU/gallon. E10 87 regular automotive gas is 114,100 BTU/gallon. So basically, 100LL avgas does not even have the energy content per gallon that today's regular E10 87 octane pump gas has.

also, a properly built and tuned 60s muscle car engine would produce much more than the 250hp they shipped with. you cannot have a properly built and tuned aircraft engine because that is not allowed. but if you were to experiment, and actually build and tune a 60s musclecar engine to properly run avgas, it would make half what is shipped with. and, the miles before overhaul would be less than 10,k. imagine maintaining a constant engine speed and egt while driving a car. clatter clatterr, boom!

 

[seaweed]

Holy carp, what a discussion! Thanks for all this enlightening information that is super interesting, yet I don’t know when I’ll ever use it…. Worth it, though, nonetheless.

 

[seaweed]

It was a pretty color of blue though!

So, is there anything that I can do to rectify this motor from running it for about 2 seasons on 100LL? Obviously changing the plugs, but short of pulling the head off for inspection, is there anything else I should consider doing to it? It’s a J3RENM, a ‘92 3hp Johnson and is doing really well at the moment after cleaning the carb and fixing a couple other issues this year after it sat for 3-4 years?

 

[poorthang]

make sure your thermostat is working, use fresh LOW octane pump gas. 87 octane with 10% ethanol is great. i would mix some marvel mystery oil. but the best thing you can do, is run that motor at operating temp for several minutes under light load and at wide open throttle. the idea is to allow the motor to rid itself of the lead and hope the rings will clear themselves. listen for detonation, and if you hear it, under light load, tear down engine and overhaul now. if you do not hear it, you should be good. i would pull plug after each run to make sure there was no detonation. little pepper specs on porcelain is light detonation. teenie little chrome balls on electrode and ground strap is dead man walking.

 

[C&K]

Just pull the exhaust cover and look in the exhaust ports. You can see the rings, piston crown, opposite cylinder wall, and the near piston skirt by looking in the exhaust ports with a flashlight. If there is excessive deposits built up in the ring lands and scuffing of the cylinder and/or piston, you can't fix it by changing fuels.

So you're gonna get people that say, oh I run it my 60's muscle car or snowmobile and it works great. Here's the problems they have not seen yet, but they will when it fails.
Avgas has a lower specific gravity than pump gas which will normally result in a lean condition without rejetting, and it has additives to reduce vapor pressure that cause it to not stay mixed with non-mineral based two-stroke oils. Avgas has ethylene dibromide in it, which reacts with lead oxide to form lead dibromide that requires an operating temperature of 200C (392 deg F) to turn to gas. Your outboard don't run that hot - it would melt it and your two-stroke oil does not mix with it.

So 100LL in aviation parlance is called a 100/130 (which it used to be called) to describe its lean burn/rich burn Performance Number (which is NOT octane). The Performance Number describes the conversion of lead oxide to lead bromide and the temperature at which it occurs to prevent lead fouling in the engine. If you want more information on this please read the technical documents for Teledyne Continental and Textron Lycoming GA aircraft engines, and this goes back to WWII when supercharged 2,000+ hp piston engines were commonly used in warbirds. But these were very short-lived engines - 500hrs and they were junk - they used to blow cylinders off 'em and lock 'em up one after another. A brand new Rolls-Royce Merlin operated at WEP (War Emergency Power) for more than 90 seconds is an automatic overhaul as soon as we get than engine back on the ground (assuming it's still in one piece).

At rich burn (full power used for takeoff and climb power settings) lead fouling occurs. At lean burn (leaned to either lean-of-peak or rich-of-peak) for cruise power settings, the temperature of the cylinder heads is set high enough to mitigate lead fouling thru conversion of lead bromide to gas. The gaseous lead bromide exits the exhaust and reforms as solid lead in the cooler air.

I hope you're getting the idea that the world of aviation engines is not even close to outboard engines. You can't run an outboard hot enough to achieve that - it will melt it.

So it's all going to depend on how many hours you put on it running avgas. If you see evidence of cylinder scuffing, piston skirt scuffing, or stuck rings by inspecting it thru the exhaust ports, tear it down now and de-carbon it and clean the cylinders up with a hone before you cause more damage. With aviation piston engines we replace cylinders and pistons all the time that don't pass a leakdown compression test on either annual or 100hr inspections. They are made to be able to easily replace a cylinder and piston by just unbolting it and pulling it off the crankcase. It's not considered an overhaul for an aviation engine - it's normal maintenance. Outboards aren't built that way and they don't reach even CLOSE to 400+ deg cylinder head temps, nor do they use lean burn/rich burn methods for mitigating lead fouling in the engine.

 

[poorthang]

i stand corrected! and have also learned from mr ck. great job with the truth mr ck!

 

[C&K]

i stand corrected! and have also learned from mr ck. great job with the truth mr ck!

Well, if you're not familiar with it, you wouldn't know. Avgas is not even like the old leaded automotive gas used to be - even 100L contains many times more lead than any automotive gas ever has.

Unfortunately, even soaking lead fouled pistons in diesel fuel won't dissolve those hard brown lead deposits. It builds up under the top piston ring until the ring is totally bound in the ring land and can't move anymore. Then when the ring can't move and flex freely in the ring land it tears the cylinder up because the ring is totally dry and no oil can reach it or get past it for lubrication. I've seen Lycoming IO-540's where an IA signed off on an annual or 100hr with the compression on one cylinder at 68/80 and it ran another 50 hrs then lost that cylinder. On teardown have to chip the top piston ring out of the land with a flat-blade screwdriver and a hammer and it was within moments of seizing the engine up.