Aircraft engine lubrication

THE theory of correct lubrication calls for the use of as thin an oil as will stay in place and maintain a film with a reasonable factor of safety. However, if such a light oil were used in an aircraft engine, it would easily get by the pistons in large quantities and into the combustion chamber of the cylinder. This is possible as it is necessary to use considerably greater clearance between the piston and the cylinder wall in an aviation engine than in an automobile engine. These large clearances are needed because the pistons run hotter due to sustained high-power output and general use of air cooling; they are nearly twice the diameter of the average auto engine piston, and are made from aluminum alloy with resulting high expansion characteristics. Consequently, automobile oil is not suitable for use in aircraft engines since it would bypass the piston too readily and result in excessive oil consumption as well as fouled spark plugs and valves.

Thus instead of selecting the lightest oil that will maintain a film, it is necessary to pick a lubricant that will not only result in economical consumption but at the same time have body enough to prevent an oil film failure. Such oils are so heavy and viscous at low temperatures that they circulate with difficulty, necessitating a careful warm-up of the engine, and oil, before take-off until a temperature is reached where the oil flows in sufficient quantities to the various bearing surfaces and particularly to the cylinder walls. Here, excessive priming may have washed away the oil film which remained when the engine was last shut off. There are, of course, other reasons why the engine must be thoroughly warmed up. The pistons, cylinders, bearings, journals and other parts having unequal expansion should be brought up to their proper operating temperatures in order to provide the proper clearances. The cylinders should also be brought to the proper temperatures in order that they and valve push rods elongate for proper valve timing.

As a rule, lubricating oil designated as suitable for use in particular aircraft engines is required to meet certain specifications made by the engine manufacturer. These include gravity, flash and fire point, viscosity, pour point, etc. The gravity of a fluid is a numerical value which is an indication of its weight. The gravity of oil is measured by means of the API (American Petroleum Institute) hydrometer. The API gravity of water is 10°, and oil which is lighter than water gives a higher value; heavier oil, lower values. By consulting a conversion table, specific gravity and weight per gallon may be determined corresponding to various API gravities. The flash point of oil is the lowest temperature at which the vapors produced by heating will ignite without setting fire to the oil itself. The fire point is the lowest temperature at which the oil itself will ignite from the burning vapor. While these tests serve to identify or classify a finished lubricating oil, they are of little value as an indication of its usefulness. Viscosity is one of the characteristics which determine how readily an oil will flow or circulate. A heavy-bodied oil is high in viscosity and slow-pouring. For a given set of operating conditions, the higher the viscosity, the higher the bearing pressure the oil film can support; the lower the viscosity, the more freely it will flow and the lower the drag on the engine parts.

Viscosity is measured with an instrument called the Saybolt Universal Viscosimeter. Tests are made at either 100° or 210° F and the viscosity number is equivalent to the number of seconds required for 60 cc of oil to flow from one container to another at the specified temperature. SAE (Society of Automotive Engineers) viscosity numbers are sometimes used in place of Saybolt. The corresponding equivalents may be found by again consulting conversion tables. For example, an SAE-50 oil is equivalent to a Saybolt 75/105 oil measured at a temperature of 210° F. The pour-point of an oil is the lowest temperature at which it will flow without being stirred or agitated, an indication of the suitability of a lubricant for cold-weather use. It naturally varies with the grade of viscosity of the oil; for Saybolt 77 it is 0° F, and for Saybolt 140, 30° F. Carbonization may be determined for comparative purposes only, but it is well-known that the heavier the carbon deposit left by the burned oil, the less suitable it is for engine lubrication purposes. In general, aircraft engine oil must be free from acid and should be reasonably transparent in a one-inch layer. It should also retain its viscosity under heat so that the viscosity when cold need not be so high as to prevent proper circulation. It has been found that good oil never "wears out," and hence becomes unfit for use only when diluted with unburned fuel or when it contains carbon, dirt or water in suspension. Even then, some distillation processes effectively reclaim such oil and again make it fit for service.

An upright in-line engine, in which the cylinders are above the crankcase, can use either a wet or dry sump lubrication system. In wet sump systems the oil reservoir is the crankcase, and a pressure pump submerged in the oil supply pumps the oil to the various parts of the engine. From these points it drains back to the crankcase sump where it is again picked up by the pump and re-circulated. The dry sump system reservoir is located outside the engine in a special tank. The lubricant is drawn from this tank by a pump and delivered to the various points in the engine. The oil drains back by gravity from these points to a small sump, where it is returned to the outside reservoir by a scavenger pump. The dry sump system also permits the use of an oil cooling system inasmuch as the oil is carried outside the engine and back again. One or more thermostatically controlled radiators help keep the circulating oil at a regulated temperature.

Research and development work by engine designers and engineers, by metallurgists and petroleum chemists and engineers, and by operating, maintenance and service personnel during the past decade have given the aircraft engine a tremendous increase in power output, with a reduction in specific weight and much improvement in specific fuel consumption. Nevertheless, these engines involve many difficult problems in lubrication. A maximum of horsepower is developed with the lowest possible engine weight, which means that a very minimum of material is used in the design of the component parts. Many surfaces are heavily loaded, and each cylinder must, on occasions, produce a maximum of available horsepower, often more than that intended by the manufacturer. There are many localized high temperatures, some of which may reach 400° to 500° F in locations where the oil flow is low, and the oil film must function without benefit of regulated change until it is destroyed. The resulting destructive decomposition must not leave residues that impede continued lubrication or the operating efficiency of the mechanism.

Extreme variations in atmospheric temperatures at the time of engine starting are encountered in any season due to the many localities in which the ship may operate daily. Such temperatures and pressures are apt to range from one extreme to another in a matter of a few minutes while in flight. This is particularly true of military aircraft which climb so rapidly into the stratosphere, often involving a temperature variation of 100° F on the ground, to -60° F at high altitudes. Atmospheric pressures vary from 14.7 lbs per square inch at sea level, to 3.5 lbs at approximately 35,000 ft. While commercial planes operate continually at very wide changes in temperatures and pressures, these are not so extreme as in the case of military craft.

The airlines have long been concerned with long cylinder barrel life, ring wear, piston lacquers, deposits and general engine cleanliness. Excessive and rapid piston ring wear often develops into ring feathering which results in excessive blow-by, shortening the overhaul life of the cylinder assembly and invariably causing very excessive oil consumption. Long cylinder barrel life or low wear is essential in keeping maintenance costs down, as the renewal of a cylinder barrel is one of the most expensive items of replacement. Excessive lacquer and sludge of a type that scales off are likely to get into the oil circulation system and clog filters and oil screens to affect the pressure and supply, causing flight interruption or delay. The lubrication of master rod bearings is of major concern, because this assembly is literally the heart of the engine. Failure often results in severe damage, or if detected before complete failure, invariably requires a complete disassembly of the engine and costly time and material expense for repair. It is probably the most sensitive unit in the engine to abrasives or foreign matter. Therefore, it is of the utmost importance that the oil supply to this assembly be kept as clean as possible and free from abrasives originating externally or internally. There should be a continuous complete flushing with filtered oil to renew the oil film and prevent any accumulation that might cause overheating and failure.

The lubrication problems of military aircraft engines often show up marginal conditions that are never detected in commercial operations. This is due to higher engine outputs and the higher rpms required in maneuvers or to meet certain emergencies that invariably result in the overheating and stressing of parts that normally give long service life. It is accordingly very essential for both the manufacturer of the engine and the supplier of the lubricant to take into consideration all the problems under the most severe operating conditions. The manufacturer should meet these requirements with design and materials that will withstand such punishment, and the oil supplier must keep pace with an oil that will facilitate the maximum of lubrication protection.

According to lubrication engineers of the Sinclair Refining Company, an aircraft engine oil of correct type and properties must meet at least twelve service requirements:

  1. minimum coefficient of friction,
  2. maximum adhesion to the lubricated surfaces,
  3. adequate film strength and oiliness characteristics,
  4. physical stability with regard to temperature and pressure,
  5. chemical stability against oxidation and thermal cracking and coking,
  6. freedom from corrosive-acid forming,
  7. resistance to combustion,
  8. non volatility,
  9. proper fluidity at low temperatures,
  10. no objectionable compounds of decomposition, with fuel by-products,
  11. correct viscosity characteristics,
  12. uniformity and purity control if additives are used.

Through hours of continuous operation the oil must maintain a factor of lubricating safety and stability. It must maintain a safe margin of protection in all cases of emergencies, such as a need to increase altitude or even climb on single engine operation, or to function well under lean or rich mixture variations. American equipment has proven that it will stand much abuse. This is not only a compliment to the experience and engineering that have gone into the ships and their power plants, but also to the quality of the fuels and lubricants which unquestionably contribute to the success of any service life.

This article was originally published in the July, 1944, issue of Air Tech magazine, vol 5 no 1, pp 30-33, 68.
The PDF of this article includes cutaway diagrams showing the lubrication systems of Wright Cyclone, Packard-built Merlin, Allison, and Pratt & Whitney Twin Wasp engines.
Diagrams credited to Sinclair Refining Co.