Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/126719
What Happens as This Happens The high-pressure oil from the pump should be a bubblefree, contaminant-free stream of oil. Modern lubricants are "long-string" molecules, containing the base petroleum stock and additives to enhance cleaning and wear, and to contain and fight contaminants such as metal, carbon, and water. As the oil is used, these molecules are literally torn apart; long-string molecules get chopped up; the oil wears out. Dr. Antika said that's not a hugely significant problem. "Viscosity loss is not typically a concern. The polymeric viscosity-improver molecules break down to some extent by shear and heat, but multigrades (certainly ours), stay [within] SAE grade during use." Much worse is contamination. Water occurs naturally through condensation. Water in oil is bad. Water is not a good lubricant. If the engine is run often enough at a high enough temperature, most "suspended" water boils off; some, though, stays as "dissolved" water and remains in the oil. The molecule—H2O—is short. This is one reason why plain water is not good in liquid cooling systems. Yes, it freezes; but even in temperate climates, and even though water's ability to absorb heat is unparalleled, it can't lubricate the water pump! Worse, water displaces oil, forcing the good lubricant out the breather and reducing the amount of lubrication available in the engine. Further, that suspended water sits on the metal surfaces, encouraging corrosion. "Additives help keep the suspended water off the metal," Dr. Antika said. Here's how: "The polar heads tend to accumulate on the metal surface; the (oily) tails repel the water." Okay, good. Carbon contamination (also bad) occurs through combustion and is blown at high temperature and pressure past the piston rings, where it is washed by the crankcase oil to the sump to be soon captured in the oil filter. Carbon is particularly abrasive, and once the rings get to a certain point in their lives and can no longer keep the combustion fully above the piston, oil filters get quickly overwhelmed and engine wear is rapid. (Hence, the periodic leakdown test.) Metal belongs where it started, but various metals are routinely and unavoidably scraped off various moving parts: steel from rings; lead, copper, antimony, and various semi-exotics from bearings; aluminum from pistons; bronze from valve guides; iron from cylinder liners—it's all in there, in varying quantities and proportions. Routine wear is predictable and unavoidable; nothing lasts forever. What we want to do is to maximize the length of this life cycle, and know if some particular wear is unexpected. When an engine is new, it is normal for the new parts to scuff each other a bit. (And as Dr. Antika noted, "Some engines are so old, they depend on deposits to keep from leaking.") As an engine "breaks in," wear declines to its operational minimum. As the engine ages, carbon and the cumulative effects of water (corrosion) and the engine's mission characteristics take over, and wear accelerates until the engine needs an overhaul. Interestingly, some good work (such as a top-end overhaul that increases compression) can escalate other wear (such as on the main and rod bearings). Rebuilding the bottom end but not the top doesn't put more stress on the top end, but this is not a normal sequence of rebuild. Regular oil analysis tracks specific contaminants and can help identify when any one of them gets off track. Likewise, the engine's oil filter should be cut open and examined at every change. When you see something there that's not "normal," it's time to find out what and why. A rebuilt, remanufactured, or overhauled engine, by the way, should generally be treated as a new engine. Use the recommended break-in oils and procedures; but beware… that first oil change may scare you. For instance, a new Rotax 912 is assembled with black (molybdenum) grease; your first oil will look like it came from a diesel. Bill Middlebrook, master rebuilder and president of Penn Yan Aero, noted, "Especially in overhauled engines, you may find silicone, glass or plastic beads from blasting, red threads from shop rags…" After one or two oil changes, that should cease. (For the record, Penn Yan doesn't bead blast or use silicone sealants in its rebuilds.) Each Component Is Critical The oil filter is effective and inexpensive. It captures particulates and even some water, but it won't do a thing to Te rod bearings are lubricated directly through the main bearings, and by splash; but this one wasn't. EAA Experimenter 29