Does this sound right? (Power and Tetraethyl Lead)

[jasc15]

I was looking up the meaning of the steps in the green arc on some tach gages. In an article I found, I read what is quoted below, which sounds pretty tenuous to me.

Lead also plays the part of coolant in the exhaust stroke of the piston aircraft engine. Since metal is a more efficient conductor of heat that the exhaust gasses, it conducts more specific heat out the exhaust and away from the combustion chamber and, more specifically, the area of the exhaust valve and valve seat.

When lead is precipitated in solid form during the combustion process, a certain amount of it is deposited on the exhaust valve seat. When the exhaust valve closes, this lead film acts as a cushion in the seat, lessening the impact of the steel-on-steel contact and reducing wear.

Finally, the film of lead deposited on the exhaust valve and seat acts as a sealant in that area, becoming a self-replacing gasket that allows greater pressure to be maintained during the next compression stroke.

Makes me suspicious of the rest of the info i read, but the explanation of the green arc steps being 75% power settings at different altitudes does sound reasonable, since some of them actually show numbers at each step.

 

[flyingron]

I don't buy it. If you're leaned anywhere close to right, the TEL is going be burned and turned into lead oxide and then scavenged by the ethelene dibromode added to Avgas to turn it into Lead Bromide. The Lead Bromide is in the deposits you find (the brownish ash on your exhaust is lead bromide mostly). It's formed at relatively low temperatures. At lower temperatures, the lead oxide gets deposited. This is what fouls your plugs and sticks and corrodes your valve stems.

The that "metal conducts more specific heat" is also completely bogus. Metals actually have lower specific heats than most of the other products of combustion. In fact any fuel or water vapor does a better job than metal oxides or salts.

 

[pericynthion]

Sounds bogus to me. TEL's main benefit is octane boost (i.e. anti knocking, allowing higher compression ratios - more important in some aviation engines than others).

The other somewhat beneficial effect it has is on the exhaust valve seats - but I don't think it is acting as a mechanical impact-reducing cushion as your quote describes, but rather a very thin film that prevents the valve from forming microscopic welds to the seat.

I use Magical Mystery Oil and Alcor TCP in my O-200 - I really don't know if they are giving me any benefit, but I started using them after having a badly stuck exhaust valve and it hasn't recurred.

 

[Ghery]

"Specific heat" isn't conducted. It is the amount of energy required to raise the temperature of a given material. I don't think I'd trust that article.

 

[dell30rb]

I was told the green arc in carbureated planes represents the area where you can (save for in precip) operate with the carb heat off.

 

[dell30rb]

Sounds bogus to me. TEL's main benefit is octane boost (i.e. anti knocking, allowing higher compression ratios - more important in some aviation engines than others).

The other somewhat beneficial effect it has is on the exhaust valve seats - but I don't think it is acting as a mechanical impact-reducing cushion as your quote describes, but rather a very thin film that prevents the valve from forming microscopic welds to the seat.

I use Magical Mystery Oil and Alcor TCP in my O-200 - I really don't know if they are giving me any benefit, but I started using them after having a badly stuck exhaust valve and it hasn't recurred.

I've seen marvel oil go into an old boat with sticky valves and clear the problem right up.

The biggest valve killer is using old gas IMO. 100LL lasts a long time but mogas does not. You can tell when its old by the smell. I have no scientific data but I would not run mogas in an airplane that is older than a few weeks.

 

[Sac Arrow]

I've seen marvel oil go into an old boat with sticky valves and clear the problem right up.

The biggest valve killer is using old gas IMO. 100LL lasts a long time but mogas does not. You can tell when its old by the smell. I have no scientific data but I would not run mogas in an airplane that is older than a few weeks.

Probably wouldn't want to run mogas in a brand new airplane either, then.

 

[Tom-D]

The biggest valve killer is using old gas IMO. 100LL lasts a long time but mogas does not. You can tell when its old by the smell. I have no scientific data but I would not run mogas in an airplane that is older than a few weeks.

How would that ruin valves?

 

[Cap'n Jack]

1) Any lead is leaving as an oxide gas, not a metal
2) I don't believe that there is enough lead to cushion or seal in modern avgas. (I could be wrong about this- Ted DuPuis really knows about this and will correct me.)

As for the comment about ethylene dibromide (technically 1,2-dibromoethane), it's not added to avgas anymore. Not enough lead to need it.

http://www.glennoil.com/msds/avgas100LL.pdf
http://www.phillips66aviation.com/pdfs/AvGas MSDS.pdf

 

[Tom-D]

I was looking up the meaning of the steps in the green arc on some tach gages. In an article I found,

That article is BS.

The only good thing TEL does for us is retard the burn rate of the fuel. stopping the detonation in the combustion chamber.

Higher Oct ratings can be obtained using less TEL than another substance.

 

[Dan Thomas]

I was told the green arc in carbureated planes represents the area where you can (save for in precip) operate with the carb heat off.

Bogus advice. Carb ice can form at any power setting, depending on engine/carb setup and atmospheric moisture and temperature. This sort of misinformation is what breaks a lot of airplanes. Reading the accident summaries will often turn up the phrase "no mechanical problems could be found with the engine or associated systems. The prevailing atmospheric conditions were conducive to carburetor icing." Far too many pilots just don't understand the who-what-where-when-why-how of carb ice and it gets too many of them. Cars had to have automatic carb heat just because drivers couldn't be trained or trusted to manage it.

The lead in fuel did have a small lubricating effect on valve seats and faces. When unleaded mogas was introduced the automakers went to harder, more wear-resistant valves and seats. http://yosemite.epa.gov/R10/airpage.nsf/webpage/Leaded+Gas+Phaseout



Dan

 

[dmccormack]

Far too many pilots just don't understand the who-what-where-when-why-how of carb ice and it gets too many of them. Cars had to have automatic carb heat just because drivers couldn't be trained or trusted to manage it.

Dan

...and pilots can?

 

[Dan Thomas]

...and pilots can?

Apparently not, for some. We could have automatic carb heat in airplanes, too, but that would cost power most of the time, and even the automatic carb heat has its limitations.

Dan

 

[dell30rb]

That's not just advice.. Its no coincidence the green arc on my 152 stops at 1900 rpm and the POH requires the carb heat be on at all times below 1900 rpm.

Well aware that carb ice can form at any power settings but you dont go running around with carb heat on at 2300 rpm unless you are flying through precip or have a reason to suspect carb ice

 

[poadeleted21]

That's not just advice.. Its no coincidence the green arc on my 152 stops at 1900 rpm and the POH requires the carb heat be on at all times below 1900 rpm.

Well aware that carb ice can form at any power settings but you dont go running around with carb heat on at 2300 rpm unless you are flying through precip or have a reason to suspect carb ice

I don't go running around in my cherokee with carb heat on at any RPM (save for the run up) unless I suspect carb ice. That's per the POH.

 

[jasc15]

I didn't want to go so far as to call the article BS at the outset (which was my first impression) for the remote chance that the author was a member here, so I was trying to be nice about it.

As for the lead thing, it's like Cap'n Jack said; it leaves in the combustion gases in compound form, not as elemental lead.

Anyway, I noticed this past weekend that there were indeed numbers on the green arc corresponding to the steps: SL, 5 and 10 (I hadn't noticed these before). I assume the part in this bogus article about the 75% power settings at those altitudes is correct.

 

[TripleZ]

From the standpoint of combustion, not all of the lead in the fuel will leave as an oxide or compound. There will always be a percentage that remains in its original form. Complete combustion is never that...... there are always residuals.

The original benefit of leaded gasoline was for octane and wear properties. It is true that leaded gas will leave a film on exhaust valve seats, guides, etc. Its beneficial in heat transfer (more from a higher surface area contact, better contact, better heat transfer) and sealing.

However.... as others have stated, the amount of metal lead deposited with low lead fuels is probably minimal and most engines have updated valve seats with harder materials so that they don't require lead.

When lead is precipitated in solid form during the combustion process, a certain amount of it is deposited on the exhaust valve seat. When the exhaust valve closes, this lead film acts as a cushion in the seat, lessening the impact of the steel-on-steel contact and reducing wear.

Finally, the film of lead deposited on the exhaust valve and seat acts as a sealant in that area, becoming a self-replacing gasket that allows greater pressure to be maintained during the next compression stroke.

This is true for leaded fuel in history....... not so much now.

Lead also plays the part of coolant in the exhaust stroke of the piston aircraft engine. Since metal is a more efficient conductor of heat that the exhaust gasses, it conducts more specific heat out the exhaust and away from the combustion chamber and, more specifically, the area of the exhaust valve and valve seat.

With better seat contact, you get better heat transfer. A soft malleable leaded seat will have better contact and heat transfer. True.

Metal better than exhaust gases....yeah.... not sure if I buy that. Better than no additive, maybe. Better than combustion residuals such as water, no.

 

[Dan Thomas]

I don't go running around in my cherokee with carb heat on at any RPM (save for the run up) unless I suspect carb ice. That's per the POH.

Piper didn't worry too much about carb ice in any of their Lycoming-powered airplanes. Lycoming's carb is on the bottom or rear of the oil sump and once the oil is hot, the carb body is plenty warm enough to make icing rare.

The previous poster's C152 POH requires carb heat below 1900 RPM. That is also a Lycoming-powered airplane, and I suspect that the advice given by Cessna there was carried forward from the old 150 POH. The 150's engine was a Continental, and those engines have their carbs mounted on a separate "spider" that's bolted to the bottom of the crankcase via a couple of studs. Not nearly as much heat is conducted to the carb body in this setup--almost none at all once the cold air/fuel mix has chilled the spider--and so carb icing is far more common in the small Continentals. It's one of those things that can bite the guy who learned to fly behind a Lycoming and then buys a Continental-power flivver and can't figure out why the RPM is falling off so bad on that nice, sunny, high-dewpoint morning.

Dan