The earlier article, "The Oil and Your Engine," was so well received that an expansion of the subject was deemed appropriate. In the previous article, we listed the two basic types of oil used in general aviation aircraft piston engines as straight mineral and ashless dispersant (AD). We also stressed the importance of clean oil in achieving good engine life; and oil consumption as an indication of engine health was another important item discussed.
Continuing our consideration of the oil and the aircraft engine, the primary purpose of a lubricant is to reduce friction between moving parts. Another additional responsibility of the oil is to help cool the engine. As it circulates through the engine, the oil absorbs heat. Pistons and cylinder walls are especially dependent on the oil for cooling. In addition to reducing friction, the oil acts as a cushion between metal parts. The oil also aids in forming a seal between the piston and the cylinder wall to prevent leakage of gases from the combustion chamber. Oils likewise help reduce wear by picking up foreign particles and carrying them to a filter where they are removed.
Using a direct drive, wet sump Lycoming powerplant as an example, we can describe the basic lubrication system of these less complex aircraft engines. A more detailed description may be found in the Overhaul Manual. In a wet sump engine the oil is contained in the engine sump as opposed to a dry sump powerplant where the oil is in an external oil tank located perhaps in the wheel well or the rear of the aircraft engine nacelle. In a dry sump engine, the oil is drawn from the oil tank and pumped throughout the engine by the pressure section of the oil pump, and then returned to the oil tank by the scavenge section of the oil pump. The other basic parts of the oil system are very similar to those used in the direct drive, wet sump Lycoming powerplants.
In the wet sump engine, the oil pump draws oil from the rear of the sump through the suction screen and sends it to the oil pressure screen. A bypass valve in some models is located between the pressure side of the oil sump and the oil screen. It permits unfiltered oil to bypass the screen and enter the engine when the oil filter is clogged, or during a cold start. The spring loading on the bypass valve allows the valve to open before the oil pressure collapses the screen, or in the case of cold congealed oil, it provides a low-resistance path around the screen. It is felt that dirty oil in an engine is better than no lubrication at all. Most oil systems offer as optional or standard, a thermostatic bypass valve in this same location which also contains a pressure relief feature to bypass the cooler in case it is clogged. As the name implies, this unit regulates the temperature of the oil by either running it through the oil cooler if it exceeds a preset temperature, or bypassing the oil cooler if the oil temperature is lower than the thermostatic by-pass setting.
Continuing its travel, the oil next encounters a pressure relief valve. The latter regulates the engine oil pressure by allowing excessive oil to return to the sump. The oil continues its travel through drilled passageways throughout the system and finally returns by gravity to the oil sump where it begins the journey all over again.
Thus the principal units in the typical wet sump engine are: a sump of sufficient size to contain the necessary amount of oil, an engine oil pump, oil cooler and by pass valve, pressure screen and by pass valve, pressure regulating valve, oil pressure and temperature instruments in the cockpit, an oil sump drain, a filler neck to put oil in the engine, a dipstick to measure the amount of oil, and a suction oil screen. The full flow oil filter is optional on the small four cylinder powerplants, but is now recommended for all engines.
We need screens and filters in the oil system to keep the oil clean as it circulates through the engine. If the oil is contaminated, it carries that contamination as it circulates. We also need an oil cooler for most engines so that the oil temperature may be kept within prescribed limits and the oil is able to perform its function efficiently. Of course, there is more to an oil system than this brief description. But for the operator who need not be a mechanic, this basic information can be helpful.
The oil companies tell us the basics about their product. Viscosity of oil is resistance to flow. An oil which flows slowly has a high viscosity. If oil flows freely, it has a low viscosity. Unfortunately, viscosity of oil is affected by high or low temperatures. At below freezing temperatures some high viscosity oils become virtually solid, which makes circulation and lubrication impossible. But no matter what viscosity oil is used, when the outside temperature is 10o F or lower, preheating a Lycoming engine is recommended before attempting to start the engine, or damage to the powerplant may result. Textron Lycoming does not approve the use of oil dilution for cold weather operation of its engines. It is extremely important that only oil in the grade recommended by Lycoming be used.
To simplify the selection
of oils, they are classified under an SAE (Society of Automotive Engineers)
system, that divides all oils into groups as follows:
If you are looking for a can of 30 weight aviation oil and it has the number 65 on it, then it is 30 and also 1065 under the Military Spec. If it has a more complete designation with the letter "W" added, then 30W indicates the viscosity (grade) of oil, it does not indicate quality or other essential characteristics. Generally speaking, any FAA approved aviation oil on the market does a good job, but it is recommended that the latest revision to Textron Lycoming Service Instruction No. 1014 be consulted to determine the appropriate grade to be used.
Multiviscosity oils have also been added to the field of aviation products. These oils cover a broad band of viscosity levels, and a number of them are approved by the latest revision to Lycoming Service Instruction 1014. These aviation grade, ashless dispersant (AD) oils have some definite advantages when used during cold weather. Because they flow more easily in cold temperatures, starting the engine is easier and lubrication of engine components gets started more quickly. Unfortunately the additives that make these oils capable of operation at all temperatures also tend to form carbon products during hot weather operation when the oil usually runs at the high end of the temperature spectrum. These products may settle out in the valve guides and contribute to sticking valves. Therefore each owner or operator should consider the type of operation the aircraft is used for and take advantage of the qualities offered by multiviscosity oils while avoiding their use if it may be the cause of potential problems.
Before we conclude our discussion, we want to remind our readers of a couple of miscellaneous but important related items. On multiengine aircraft each engine is supplied with oil from its own complete and independent system. Another reminder — every engine has a breather that can be considered a part of the oil system. If the engine does not have special provisions for aerobatic flight, and is flown inverted, the oil will be lost out the breather and a serious engine failure may result. We definitely recommend that engines not built for aerobatic flight should not be flown inverted.
In summing up this brief supplemental bit of information about the oil and your engine, remember that it was not intended as instruction as detailed as the knowledge required of a mechanic. But we think you will agree that the "Typical Pilot" should know more about his aircraft engine than "Mr. Typical Driver" knows about his automobile engine—the penalty for not knowing the basics is greater in aviation.