6.2.3 Engine Lubrication
Function and Methods
The primary function of the engine oil system is to reduce friction between moving parts which would otherwise generate heat if not sufficiently lubricated. Other functions include:
• Cushioning effect to engine parts subject to shock-loading
• Aids as an effective cooling agent (along with air cooling)
• Removing heat from the cylinders
• Providing a seal between the cylinder walls and pistons
• Carrying away contaminants
• Operation of the propeller Constant Speed Unit (C.S.U)
Viscosity describes the resistance of an oil to flow and is primarily affected by temperature. Low temperatures increase viscosity (stickiness), creating a dragging effect, hindering its ability to circulate and perform as it should. At high temperatures, viscosity decreases and the oil becomes so thin that it begins to break down, resulting in rapid wear of moving parts. Because reciprocating engines have high operating temperatures and pressures, we require high viscosity oil. Other qualities of suitable lubricating oil include:
a) High flash point (temperature at which flammable vapors are released)
b) High anti-friction characteristics
c) Maximum fluidity at low temperatures
d) Maximum anti cooling ability
e) Maximum resistance to oxidation
f) Be non-corrosive
Lubrication of the engine parts take place in one of two ways. Either by means of high or low-pressure distribution throughout the various moving parts, or via splash lubrication which results from the moving parts picking up oil and ‘splashing’ it around. The latter being considerably less effective.
Methods of Oil Circulation
Reciprocating engines use either a wet-sump or a dry-sump oil system. In a wet-sump system, the oil is located in a sump that is an integral part of the engine. Whereas a dry-sump system makes use of a separate, self-contained oil tank and engine driven pumps to achieve circulation.
Oil Pump, Filter and Sumps
The main component of a wet-sump system is the gear-type oil pump, which draws oil from the sump and routes it to the engine. Located before the oil pump is the by-pass valve which allows unfiltered oil to enter the system in case of any blockage. Similarly, an oil pressure relief valve ensures pressure is neither too high as to allow leaks, nor too low so to ensure adequate lubrication. After the oil passes through the engine, it drains back into to the sump, completing the cycle. In some engines, additional lubrication is supplied by the rotating crankshaft, which splashes oil onto portions of the engine.
An oil pump also supplies oil pressure in a dry-sump system, but the source of the oil is located in a separate oil tank. After oil is routed through the engine, it is pumped from the various locations in the engine back to the oil tank by scavenge pumps. Dry-sump systems allow for a greater volume of oil to be supplied to the engine, as well as inverted flight, which makes them more suitable for aerobatic and turbine aircraft.
Quantities and Grades
Oils are classified under seven Society of Automotive Engineers (SAE) groups – SAE 10 to 70.
An SAE number is an indication of the viscosity/grade only. The prefix letter “W” indicates that the oil is suitable for winter operations. Aircraft engine oils are often referred to by grade instead of SAE numbers, for example D100, or W100.
Temperature and Pressure Control
The oil pressure gauge provides a direct indication of the oil system operation. It measures the pressure in pounds per square inch (psi) of the oil supplied to the engine. There should be an indication of oil pressure during engine start. Oil pressure should be kept within the limits. Refer to the Pilots Operating Handbook (P.O.H) for manufacturer limitations.
the oil temperature gauge measures the temperature of oil. A green area shows the normal operating range, and the red line indicates the maximum allowable temperature. Unlike oil pressure, changes in oil temperature occur gradually. This is particularly noticeable after starting a cold engine, when it may take several minutes or longer for the gauge to show any increase in oil temperature.
Oil Cooling Methods
Since changes in temperature significantly affects the viscosity of our oil and therefore its effectiveness, an oil cooler which is placed in the airflow (similar to a radiator) and allows for oil temperature regulation by means of circulation.
Recognition of Oil System Malfunction
The oil filler cap and dipstick (for measuring the oil quantity) are usually accessible through a panel in the engine cowling. If the quantity does not meet the manufacturer’s recommended operating levels, oil should be added. The POH or placards near the access panel provide information about the correct oil type and weight, as well as the minimum and maximum oil quantity. Within the filler neck is an oil filter to prevent foreign particles entering the engine compartments. At the bottom of the sump is a quick drain valve to manually remove water or sludge. Checking oil quantity is part of your pre-flight checks and should be done prior to every flight.
It is important to also periodically check the oil temperature and pressure gauges during flight, especially when operating in high or low ambient air temperature:
• High oil temperature indications may signal a plugged oil line, a low oil quantity, a blocked oil cooler, or a defective temperature gauge.
• Low oil temperature indications may signal improper oil viscosity during cold weather operations.
• High oil pressure could mean possible internal plugging or cold oil.
• Low oil pressure might indicate a broken pressure relief valve, insufficient oil or burnt out bearings.