Alternative oiling schemes
Aircraft-engine lubrication systems haven't changed much through the years. Yes, there has been a general move toward more sophisticated filter systems — with full-flow cartridges taking the place of less than thorough screens — and the oils themselves are far improved, with multiviscosity additives for petroleum-based lubes and the advent of synthetics.
But the basic nuts and bolts of the aircraft oil system have changed little, and that means the engine-driven pump only begins producing oil pressure after the engine has started. Seems natural enough, but therein lies a problem. If the airplane has been sitting unused long enough, much of the oil surrounding the bearings and protecting the metal surfaces from corrosion may well have stripped off and settled down to a leisurely life in the oil sump. In those first few moments of activity, the crankshaft and camshaft bearings, as well as the contact points between the cam and lifters, among the teeth of the accessory gears, may well be without lubrication.
Believing that the potential for excessive start-up wear could be easily reduced, George McCrillis has packaged and obtained approval for installation of something called a preoiler. In concept the Oilamatic Preoiler (company telephone 303/770-0175), is simple. A small electric motor, connected to a miniature oil pump, takes lubricant from the engine sump and sends it under normal operating pressure to the engine's galleries. In theory, this will ensure that all the bearings are fully wet with oil before the engine is even so much as primed for the start. You will see green-arc oil pressure indications before turning the key.
McCrillis cites research that shows some 70 percent of engine wear takes place at startup. (This statement surely takes into account all post-start stresses, not just those attributable to lack of pressure lubrication; we'll explain in detail shortly.) His system is available for a wide variety of airplanes, single and multiengine; prices range from $1,145 for a single-engine installation to $2,395 for a twin.
Potential benefits of the preoiler vary. But it helps to understand something about the oiling system to know why. In its most basic form, the lubrication system is responsible for routing filtered oil to the high-stress areas within the engine to cool and protect the parts from metal-to-metal contact. The idea is to form an oil film between materials, which reduces the friction and wicks away heat that would otherwise have to be shed through the mounting apparatus itself.
As with most modern engines, aircraft powerplants use predominantly plain bearings to locate the major components, the crankshaft, camshaft, and accessory gears. These bearings are made up of sandwiched metal, generally softer than the material rotating within them, which conform to the curvature of the part. They fit into like-shaped recesses in the engine cases.
At the turn of the century, when ball or roller bearings were common in engines, low-pressure lubrication systems worked sufficiently well that total-loss schemes were often used. That is, oil was dripped onto a part from a reservoir; there was no attempt to recapture this oil and reuse it inside the engine. Roller bearings are expensive to produce, however, and preclude constructing a crank or cam from a single piece of steel, because the bearing would have to be pressed into place during the shaft's construction.
As mentioned, the oil pump is driven by the engine, usually from the accessory gears at the rear of the engine. (These same gears drive the camshaft from the crank and motivate the magnetos, vacuum pumps, and, in some cases, alternators.) In some applications, principally turbocharged powerplants, there is another pump, called the scavenge pump, that draws oil out of any lubricant-served accessory, such as a turbocharger.
Oil will not forever remain on internal parts and within the galleries, which route the slippery stuff to the crank and cam bearing journals, hydraulic tappet bodies, and accessory-case journals. The potential, then, is for surfaces that are asked to spin immediately after engine start do so without the benefit of the proper oil film. (This is particularly true in cold weather, which only should reinforce that a thorough preheat ought to be conducted. When? Opinion varies, but most agree that anything below freezing warrants external preheat, though many make the cutoff at about 20 degrees Fahrenheit.) For these components, a preoiler will likely benefit longevity.
But not all of the engine gets lubricant from a pressure source. You might be surprised to know that the camshaft has no direct oiling for the faces of the lobes or the bases of the lifters. These components depend on splash lubrication, a fancy way of saying that oil slung from the spinning crankshaft adheres to the camshaft and lifters. Likewise, the cylinder barrels have no direct pressure oiling, requiring the good services of the crankshaft to place oil in the barrels, where the rings eventually drag it around the bore. (Many automotive and motorcycle engines use oil spray jets threaded into the connecting-rod big end to coat the underside of the piston with cool oil and to apply the lubricant to the cylinder walls; alas, relatively few aircraft engines use this scheme.)
Cylinders, lifter faces, and camshafts, then, do not greatly benefit from preoiling. Some advantage may be gained in having the fresh oil at hand on the crank journals thanks to the preoiler, but merely running the system pressure up to operating specs will not ensure that the cam and cylinder walls are suitably coated with oil. (Differences in engine construction suggest that under-crank cam locations, found in Continentals and the rare Lycoming, may gain from the oil oozing onto the surfaces from the weeping crank above. These dissimilarities are supported by the generally higher incidence of camshaft and lifter wear in top-cam Lycomings.)
In realizing that even normal operating oil pressure doesn't ensure good camshaft and lifter lubrication in Lycomings, another inventor, Chuck Ney (telephone 800/648-6917) set about designing a fix. Now approved for installation on just about every Lycoming on the planet, the Ney Nozzle is a wonderfully clever device. Installed at overhaul time, small nozzles are threaded into the cases adjacent to the lifter oil galleries, allowing pressure lubrication to be aimed directly at the cam and followers. This modification sells for $245 and must be performed during a major overhaul because the cases must be split and some welding accomplished in addition to the machining.
Ney's modification should be of particular interest to owners of airplanes with certain Lycoming engines, specifically the 76 series, which are known to be camshaft and lifter consumers. As installed in 1977 through 1980 Cessna 172s, the O-320-H2AD, along with its O-360-E models, is a substantially different engine compared to other Lycomings. Typically Lycoming uses mushroom lifters, whose faces are substantially larger than the bore the lifters ride in; this allows for greater contact area with the camshaft. Unfortunately, it also means you can't pull the lifters without splitting the cases. The 76 engines use barrel lifters, so familiar to mechanics working on Continentals. In addition, theses models placed the camshafts farther from the crank, which could mean a reduction of splash-oil lubrication.
It is too early to tell if the Ney Nozzle, either alone or in conjunction with a preoiler, will cure these Lycomings' appetites for camshafts. But if a good idea gets noticed, then Ney's qualifies — Engine Components, one of the largest crankcase repair facilities in the U.S. has signed on as a licensed seller of the Ney Nozzle. Cases requiring work during overhaul are frequently set off to shops like ECI, and considering the total cost of the job, an additional $245 spent on the case is the proverbial drop in the bucket.
