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Wed, Apr 23, 2003

ProSpeak: Piston-Engine Aviation Oil

Lubrication and Rust Prevention -- It Does Two Big Jobs

by Tim Kern

It's always daunting, when we journalists run into someone so much smarter than ourselves, that we can't even pretend to belong in the same room with him. It's even better, when we get invited in. Such was the case at Sun 'n Fun, when Exxon Mobil Aviation Lubricants Product Advisor, Shlomo Antika, sat down with us, to give some insight into what aviation oil becomes, once it's in our engines.

Dr. Antika (he's a PhD. Chemical Engineer) started with some basics: without proper lubrication and rust protection, our engines rust out, or wear out. Although the two conditions are obviously related at times, the challenges for our oil are quite different.

"You have to have oil," he said, starting slowly. "You need the proper mechanical conditions, and a proper filter. The oil has to be of the right viscosity; and it has to be the right type of oil. Nowadays, that usually means a dispersant type."

In older engines, he explained, mineral oil (non-dispersant) may still be what you want. "You're looking at 20~25 hour oil changes," he noted, "and you're probably losing or burning a fair amount [of oil], so it's really not too bad, internally [inside the engine]." One other fact of life comes into play: "Some engines are so old, they depend on deposits to keep from leaking." If that's your engine, keep the oil clean and change it often. You can go to a dispersant oil at your next rebuild.

"All oils," Dr. Shlomo said, "require a basic level of filtration -- you have to remove the dirt, contaminants, and byproducts of fuel degradation." Gas in the oil is a multi-faceted problem: it thins the oil, and it can create a more-combustible environment inside the engine.

"The only way to get gas out of the oil," Dr. Antika noted, "is to run it out, or, of course, change the oil. This is not a 'filter' issue."

His job, on the 'lubrication' side, sounds simple: "What oil engineers do is create molecules that get between the moving metal parts."

Moisture, bad; lubrication, good.

"You must protect your engine against wear and rust. Both take place on steel surfaces; wear occurs on any potentially rubbing surface."

"Wear additives," he continued, "protect the moving parts with a molecular layer [of lubricant and metal surface strengthened by reaction with antiwear additive] between them."

Lube requirements: thin enough, thick enough, slick enough, tough enough.

Oil has to be thin enough to flow, at the lowest temperature your engine encounters. That's at startup; and the temperature is dictated by the environment. A low-viscosity oil (a 5W, for instance) will flow better at lower temps than a higher-vis (40) will. A multi-viscosity oil, like a 20W-50, will flow like a 20W at low temps.

"At low temperatures," our host said, "the oil must be able to flow. Some engines are so tight mechanically, that's a real challenge. There is 'no' mechanical room for the oil to flow -- another reason to do a careful warmup."

At high temperatures, there's another set of problems. They relate to the lubrication properties. Will the oil's molecular structure break down and allow metal-to-metal contact? "Is the viscosity sufficiently high to protect the parts? You have to maintain a lubrication layer. This layer -- it may be only 20 microns -- but it has to be there," said the engineer. "You don't want metal to touch metal." As temperatures go up, the lubrication problem becomes greater. "If you use even a 40-grade oil in a hot engine on a hot summer day, you may not have sufficient lubrication."

Rust is often the bigger enemy of your engine.

"Rust prevention," the good doctor said, "is 'formulation science,' a pseudo-science,' really." That means there are elements of art and experience, and plain old trial-and-error involved. There is a popular misconception, that 'rust' and 'corrosion' are pretty much the same thing, differentiated only by the metals involved. For the purposes of our understanding, that's not so. "Corrosion is a chemical attack on the metal, due to additive or oil degradation. It's typically seen in higher temperatures. Water is not part of this phenomenon," Dr Antika explained. "Rust, on the other hand, involves water on ferrous metals. Technically, you're talking about 'galvanic corrosion,' a battery formed in the presence of water on iron."

"Any oil, even mineral oil," Dr Antika said, "will give you some rust protection. In a formulated oil, you also have chemistry on your side, as well as the physics of the oil."

Two kinds of water?

"There are two kinds of moisture we talk about in oil," he told us. "There's 'dissolved water,' and 'suspended water.'" The only way to get rid of the dissolved water is to change the oil -- but the amount of dissolved water, as a proportion of the moisture in your engine, is miniscule, and really not in the same league as suspended water, as a trouble-maker. Address the suspended water, and let the dissolved water take care of itself.

"Suspended water," he continued, "is bad. It sits on metal surfaces. We design additives to keep the suspended water off the metal." Here's how: "'the polar heads' tend to accumulate on the metal surface, the (oily) tails repel the water."

Here's how it works: "Oil picks up pollutants during operation and when it sits. As an engine runs, it produces byproducts of combustion and wear -- the filter handles those." Water is the bigger problem: "Water -- especially if you fly infrequently and/or in a humid environment -- you accumulate water, and you often don't get enough 'high-temp time' to get rid of it." Suspended water will boil (surprise!) at water's boiling point. "You have to exceed that temperature for some time," Shlomo explained, "and there has to be an area through which the water can escape. Some water condenses and returns: that's why the time of high-temperature operation is important."

How hot is 'hot enough?'

"If your oil temperature gauge says 190F," he picked a typical example, "it's likely the working surfaces are 220, 230 -- it's at those surfaces where water will flash off. If you run long enough, hot enough, you'll get rid of the suspended water. That's the best you can do."

"At even 220~250 degrees, oil degradation is not really a concern, if you're using a properly-formulated oil, for 100, maybe 200 hours. Higher temperatures, of course, accelerate the process..." Change your oil: "You change oil at 50, 100 hours -- an internal combustion engine is rough on oil; it produces a lot of combustion byproducts."

What about oil heaters?

If 'hot' is good, is a heater better? Like everything else a good doctor will prescribe, moderation is the key. "People use oil heaters to help with the flow at startup. That's fine," said our mentor. "But when you leave the heater on all the time, you'll be percolating the oil in the pan. Oil will drain off the static surfaces -- yet you're still percolating the water in the oil in the pan," he warned. "That water will condense on the exposed surfaces; and much of it will just return to the pan -- but the rest will stay on those exposed surfaces." Without a way to escape, the water just stays in the engine -- but now, the temperatures are higher, and the chemical reactions are thereby accelerated. "Additives will help maintain a barrier to the water," he said, but why stress them more than you have to?

As far as it goes, a heater can be a good idea, Dr. Antika said, but "Turn it on a couple hours before flight; don't leave it on all the time, or even overnight." [It depends on the heater, of course. The Tanis system heats the whole firewall-forward area, and avoids these typical problems. Turn the Tanis on all night; it'll work --ed.]

In summation:
  • Whenever possible, use a high-quality, dispersant oil
  • Make sure the viscosity range is proper for your flying and climate
  • Use a pre-heater, but sparingly
  • Fly long and often [great idea, eh?]
  • Employ a quality filter
  • Have your oil analyzed, or at least cut open the old filter and have a look
  • Change both oil and filter frequently -- both are cheaper than metal
  • If you're not going to be flying for a while -- change the oil and filter, run it for a few minutes, then park it.

[This article is the second in a series (here's the FIRST) of regular ANN features, wherein we periodically bring you tips and advice from professionals in the field, of interest to our readers. We'd like to know what you think of this format, and this idea -- and this article. Excuse us, now -- we've gotta get out to the hangar...]

[Thanks to Exxon and Tanair for the photos --ed.]



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