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I'm aware of the issues of spark plug fouling and lead deposits that tetraethyl lead can have on an aircraft engine, but what effects do lead and its compounds (metallic Pb, PbO, PbCl2, PbBr2) have on a piston engine's oil? Are their special additives that need to be added into aircraft engine oil to deal with the lead? Does the oil become more acidic or something as a result of the lead and compounds? I can imagine that lead causes quite a significant amount of deterioration to an engine's oil.

I'd also like to know if anyone knows, or can make an educated guess on, whether or not emissions of hydrocarbons, NOx, CO, and the like can be reduced by the use of unleaded avgas.

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  • $\begingroup$ The impetus for going unleaded in cars was originally to put catalytic converters in, so the answer to your second question is yes. $\endgroup$
    – user71659
    Dec 1, 2023 at 19:43

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Yes theoretically you could put catalytic converters on airplanes running unleaded avgas and reduce those compounds. It isn't going to happen because the overall impact is too small to make up for the hassle, unless an engine was designed from the start to have a CC, which you may very well see sometime in the future (just not on existing engines with carburetors or mechanical fuel injection).

Aviation oils use ashless dispersant additives to hold combustion byproducts in suspension in the oil. It's what the AD stands for in the oil type. Without AD additives, engines would be coated in gum and sludge internally when you take them apart. ADs make the engine internals clean enough to eat off of when you disassemble them

As far as lead goes, the ONLY benefit to lead in an aircraft engine other than the octane rating is it forms an oxide coating on the exhaust valve seat that lubricates the valve and seat interface as the valve closes (valves are never perfectly centered, and there's always some shifting as it contacts one point on the seat first and has to center itself, so there is always sliding contact).

Cars went to hardened valve seats (made from an alloy called Stellite) when unleaded was introduced in the 70s to avoid higher wear issues. I believe most modern A/C engines and overhauled cylinders also use hardened seats, and airplanes running unleaded Mogas seem to have no problem with exhaust seat wear.

Beyond that, lead combustion byproducts are nothing but grief in an engine, creating nasty corrosion compounds in the oil, and everything depends on the ability of the oil to hold those compounds in suspension. Besides the well know problem of spark plug fouling, lead compounds also eventually start to collect on the valve stem and jam up the guide, making the exhaust valve stick (it first shows up as really rough running while cold then suddenly smoothing out as the valve unsticks - once it really jams up it can bend a push rod and increase your stress level quite a lot as the engine starts running minus one cylinder).

PAO (polyalphaolefin) synthetic oils, in spite of their superiority as lubricants, can't be used in piston aircraft engines running leaded fuel because the PAO hydrocarbons can't keep lead byproducts in suspension even with dispersant additives (the hydrocarbon molecules are too "smooth").

Mobil learned this the hard way with AV1 full PAO synthetic oil and there was a huge class action lawsuit when the oil was introduced in the late 80s, followed by a "lead paint" crisis and a number of engine failures from plugged oil galleries (Mobil thought their dispersant package would keep lead compounds in suspension; it didn't'. It turns out the base hydrocarbon needs to help, and PAOs do a terrible job at it).

I run my own airplane on Mogas and I would use AV1 now it if was available, but I use the next best thing, Aeroshell 15W50 which is half PAO synthetic (basically, enough mineral oil to make the dispersant additives work).

Once unleaded avgas replaces the leaded version, I expect Mobil to reintroduce AV1 and wouldn't be surprised to see the latest piston engines with FADEC fuel injection systems starting to introduce catalytic converters.

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    $\begingroup$ excellent explanation!!! -NN $\endgroup$ Dec 2, 2023 at 20:21
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    $\begingroup$ Thanks. I used Mobil AV1 briefly in my '68 Cardinal in 1989, but stopped after a couple of change cycles after learning of the lead paint problems. $\endgroup$
    – John K
    Dec 3, 2023 at 5:19
  • $\begingroup$ (1) I'm assuming "lead paint" refers to sludge consisting of lead and lead compounds. (2) I always found it strange why airplane engine oil didn't go fully synthetic, but after learning about the fact that the oil's base stock needs to have good solvency in order for dispersant additives to clean up lead combustion products, it makes sense. (3) I can see the ability to finally use fully synthetic oil being another good driving force behind the push towards unleaded avgas. $\endgroup$
    – user26259
    Dec 11, 2023 at 14:16
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    $\begingroup$ 1. Yes the sludge would coat the inside of the crankcase with a tough paint-like lead coating. 2. PAO hydrocarbon molecules are smooth like wet spaghetti noodles, why they lubricate better, whereas mineral hydrocarbons are like bits of string with fuzz on them & it's the "fuzz" that helps the dispersants keep the contaminant molecules bound to the oil. What sold me on Aeroshell 15W50 is the 50% PAO is sufficient for the oil to achieve its wide viscosity range without viscosity index improvers, which don't contribute to lubrication, and wear out. 15W50 also has the anti-scuffing additive (TCP). $\endgroup$
    – John K
    Dec 11, 2023 at 19:49
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    $\begingroup$ @ElEctric yeah something like that. But apparently Mobil thought they had the problem licked in testing. But as I learned in my old tech support engineering job, testing and the real world are two different things. $\endgroup$
    – John K
    Jan 16 at 5:11

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