OT leaded aviation fuel

[Commander_Chaos]

I just read an article about the EPA's failure to get rid of leaded aviation fuel but they were really vague about why airplane engines need the additive.

I used leaded fuel when I was racing, but I don't really know what it did. I know it keeps valves from sticking to the seats in old engines, but the bikes I was racing were much newer.

Somebody here knows, right?

[darryl]

Yes

[Commander_Chaos]

Yes

What's your problem?

[Captain K]

I believe the lead was originally and as a knock prevention. You could burn lower quality (octane) fuel without causing pre ignition and knocking in your engine. Higher octane fuel has a higher ignition temperature, allowing a higher compression ratio without pre ignition. Any benefit to the valves was coincidental as I understand it.

https://en.m.wikipedia.org/wiki/Tetraethyllead

[Doc Nickel]

That's primarily it- to lubricate the valves.

You have to remember that current aviation piston-engine technology is firmly rooted in the 50s. Carburetors, magnetos, cylinders and heads based off of WW2 radial technology, etc.

To get any change approved- like different valve seats, or different valve materials- is a long and costly process. We should have modern computerized spark ignition, cutting-edge EFI, and maybe even sodium-filled valves and the like, but no one has the money to push something like that through the approval process, especially given the limited number of sales that are even possible, let alone probable. Lots of pilots might want a proven EFI system, but not if it's going to cost $9,000.

As such, we're stuck with the valves and heads from when most of the light aircraft engines were made- the 50s. To reduce seat erosion, these engines basically have to have an additive- and tetraethyl lead is the best of the lot. (Performance wise, not necessarily health wise.)

So the aircraft lobby got an exception- they kept the lead and get the higher octane.The lead is both a valve seat lubricant and an octane booster, so you get both traits from the same stuff.

For racing, the additional octane lets you run more spark advance for more power, but without getting knock.detonation. And for older engines that still run steel valves on iron seats, the lead helps drastically reduce valve erosion.

Those really are the only reasons to keep it around.

Doc.

[J Tiers]

Maybe the best way to put it is not really about what the lead does, but the huge "installed base" of engines designed to use it. The "design requirement" might be due to octane requirements, or valve lube issues etc.

Or it may nor necessarily be for any of those reasons, but because the engine is placarded to use standard leaded fuel, and nobody knows what would happen if the leaded fuel were NOT used.

Aircraft engines are really very "old tech", many are older and not generally very sophisticated, at least in the "general aviation" category of gasoline piston engines. But they are very well documented and rather expensive old tech.

There is an understandable reluctance to make a change that you do not know (and would have a pretty hard time proving) will not cause many engines to "shell out" after say 350 hours, probably on rainy nights with crap visibility over terrain that is rough to land on.

Nobody wants to be the one that said it was fine, only to have the accident rate go through the roof as engines throw cylinders and quit all over the country with a death rate sky high. Even if they just throw rods on startup (at 40 grand per each) that would be pretty bad.

It's a reasonably low consumption usage, letting it ride is a much safer approach.

[darryl]

I can imagine also that before modern fuel injection the mixing of fuel and air wasn't particularly good, so there needed to be more help against detonation.

[tom_d]

That's primarily it- to lubricate the valves.

You have to remember that current aviation piston-engine technology is firmly rooted in the 50s. Carburetors, magnetos, cylinders and heads based off of WW2 radial technology, etc.





Doc.

Close. More like technology firmly rooted in the 20s.

Today's aviation fuel is 100 octane. Sometimes referred to as 100LL, or "100 low lead", the maximum amount of lead is around 2.12 g/gal if I recall correctly. Any replacement fuel would have to be compatible with every single piston engine aircraft flying today. That would include the ones built in the 1920's. There is ongoing research to find a good replacement for 100LL, and there are a few in trial stages now. Problem is that it's a tall order to get something affordable that will serve as a universal replacement. Within the past few years it's become possible to replace one of the engine magnetos with a modern electronic ignition system. This makes for improvements in the overall operating efficiency of the engine, and would go a long way towards eliminating the need for 100LL, but things are slow to change with an entity the size of the FAA, and not every engine will accept the electronic system. An issue at hand is that the magneto supplies its own spark, meaning it does not need an outside electrical source such as a battery or generator.

The lead is needed to both maintain the octane rating and to provide a small amount of lubrication to the exhaust valves. Because these engines have very basic magneto ignition systems there is no way to adjust the spark timing to correspond with engine speed. Once the timing is set on the ground during installation, and checked during maintenance inspections, it stays fixed. Only variation will be a slight change from cam lobe and breaker point wear. During operation, from sea level up to the aircraft's max altitude, the only way to keep the valves from burning is to adjust the fuel air mixture. The fuel is not just a source of energy, it's an integral part of the cooling system too. A slightly rich mixture is used to control pre-ignition and detonation issues, things that would be a serious problem with faster burning lower octane fuels. Additionally, 100LL will not hold any water in suspension. Having no water in your fuel becomes an important consideration at higher altitudes where the temperatures are always below freezing, even in the summer months. And there are times when, if you go high enough, you'll see -40F.

[The Artful Bodger]

Because these engines have very basic magneto ignition systems there is no way to adjust the spark timing to correspond with engine speed. Once the timing is set on the ground during installation, and checked during maintenance inspections, it stays fixed.

Not totally true as there has been magneto ignition systems for advance and retard for the last 125 years or so.

Almost all (?) spark ignition aircraft engines retard the ignition during cranking.

[10KPete]

The old, simple, systems are, these days way more reliable and easier to fix than any electrickery could dream of. Vacuum pump on the engine, etc, for when the electrons fail to obey. That is the top of my list. And add on from there.

These engine run at one speed most of the time so there is no need for vacuum advance stuff as the mechanical easily does the job. We do play with the mixture but again most running is at the same setting.

As my Dad explained it 60 years ago "It's a stationary, air cooled fan motor. Totally different needs from a car."

...miss flying very much.

Pete

[Ian B]

Tom_d,

I know what you mean about "Any replacement fuel would have to be compatible with every single piston engine aircraft flying today", but how did the car world do it? They must have also used the same argument against eliminating lead (and the fact that the Ethyl Corporation would lose millions had a lot to do with it I guess).

One option would be to have more than one grade of fuel, as is done for cars (I can pick from E15 (95 octane +15% ethanol) or E5 (98 octane + 5% ethanol) (and diesel of course, sometimes LPG. occasionally 102 octane, but rare). Costs would go up a bit probably.

Another way may be to add a lead replacement: https://www.holtsauto.com/redex/prod...ent-multidose/ but this would no doubt involve type approval etc.

I know that older car engines can have their valve seats replaced with harder metal, after which they can use unleaded fuel. Also an option maybe?

If the aviation industry don't respond, they may be the next target of the Eco's, possibly flight bans over cities etc. Far fetched? Try entering many German cities with a pre-2009 diesel: https://www.reuters.com/article/us-g...-idUSKCN1NK28L Millions of cars & vans are affected by this, not just a relatively few light aircraft.

Ian

[fjk]

Tom_d,

I know what you mean about "Any replacement fuel would have to be compatible with every single piston engine aircraft flying today", but how did the car world do it? They must have also used the same argument against eliminating lead

They did
but if cars have problems and the engine quits, you roll to the side of the road, not fall from the sky, so the safety argument didn’t apply (plus at the time we were less terrified of risk and “something might go wrong”)

and

when lead was removed, cars didn’t last as long as they do today, so there was no installed base problem.

[Ian B]

I think that lead was removed from petrol in 1996, so only 25 years ago. Around this time, the majority of cars had both electronic ignition and fuel injection. That year, there were around 670 million cars on the road. Sounds like quite an installed base to me.

I don't know how many light aircraft there are (is there a strict definition of what one is?), but of one of the more popular ones, the Cessna 172, less than 50,000 have ever been made to date. How many of all light aircraft are there? Dunno - 1/2 million maybe? Just guessing here.

Ian

[A.K. Boomer]

I think that lead was removed from petrol in 1996, so only 25 years ago. Around this time, the majority of cars had both electronic ignition and fuel injection. That year, there were around 670 million cars on the road. Sounds like quite an installed base to me.

I don't know how many light aircraft there are (is there a strict definition of what one is?), but of one of the more popular ones, the Cessna 172, less than 50,000 have ever been made to date. How many of all light aircraft are there? Dunno - 1/2 million maybe? Just guessing here.

Ian

25 years ago? no - way, way before that...

lead was an old school anti-knock, and in vehicles ethanol is one of the more modern much more cleaner replacements but all modern day vehicles have stellite valve seats or something similar in durability, heck the old V/8's used to just cut the valve seats right into the cast iron head --- and get away with it when lead was used,,,

Lead is best described as a valve seat "cushion" instead of lubricant,,, but with every closing of the valve against the 45 degree angle and then combustion pressures there is a slight fretting as the valve gets jammed down even further than just spring pressure alone, so lead actually does accomplish both goals...

There's all kinds of reasoning as to why aircraft have not made the switch over,,, first off cars are unique in their "congestion" factor and cat. converts will be destroyed with lead, so absolutely mandatory to never run leaded fuel in a modern vehicle...

Aircraft are immune to the congestion rules and regs, so first off no catz required, with no worries about lead in that respect and it also being an added safety factor for all the older aircraft engines it's hard to come up with sound reasoning to make the switch-over and demand everyone tear down their current engines and have them re-worked with more durable valve seat material....

[Ian B]

I was also surprised: https://www.eia.gov/energyexplained/...0in%20vehicles.

This was the point at which it was completely phased out. But indeed, unleaded was first available in the 1970's.

You have a point about making the switch-over. I guess the car industry did this by making new models suitable for lead-free, and over the decades, the older ones faded away until they became the small minority. The same is likely to happen with electric (and hydrogen?).

Make lead free fuel available, and the market for it is likely to follow. After 5 years, increase the tax on 100LL. A market would likely then develop for replacing valve seats in older engines (it did for cars).

It would also be a great opportunity to drag aircraft engine designs into the 21st century; fuel injection, electronic fuel ignition, 4 valves per cylinder, liquid cooling etc. One ignition per spark plug, no more HT leads. Higher specific outputs (in a world where 150BHP from a 5 liter engine is seen as "normal"). Even, shock, horror, only 1 spark plug per cylinder

Ian