Engine Lubrication Systems

16.1 Functions of Lubricating Oil

• While the primary task is to reduce friction and wear between moving parts, the oil also
• performs several critical secondary functions:
•  Cooling: It absorbs and carries heat away from hot components like pistons and bearings78.
•  Cleaning: It carries away contaminants like carbon, sludge, and metal particles, which are
• then removed by filters79.
•  Hydraulic Medium: It can be used to power components like variable-pitch propellers80.
•  Condition Indicator: Oil pressure and temperature gauges provide vital information about
• the engine's health81.

16.2 Lubrication System Types

•  Wet Sump System: The oil is stored in the sump at the bottom of the engine. This is simple
• and common on light, non-aerobatic aircraft but can cause lubrication problems during
• maneuvers and makes oil cooling more difficult82828282.
•  Dry Sump System: The oil is stored in a separate external tank, and oil is returned from the
• sump to the tank by a scavenge pump83. This allows for a larger oil capacity, better cooling,
• and is suitable for aerobatic flight. The scavenge pump must have a larger capacity (25-50%
• more) than the pressure pump to prevent oil from accumulating in the sump84.

16.3 System Components

•  Oil Tank (Dry Sump): Contains the oil supply and includes a hot pot or inner compartment
• to allow a small portion of the oil to warm up quickly on startup85.
•  Pumps: Gear-type pumps are used. The

• pressure pump supplies oil to the engine, and the scavenge pump returns it to the
• tank86868686.
•  Filters: Coarse suction filters protect the pumps, while fine pressure filters remove small
• contaminants from the oil before it enters the engine87878787.
•  Pressure Relief Valve: Prevents excessive oil pressure by returning excess oil to the inlet
• side of the pump88.
•  Oil Cooler: A radiator that uses ram air to cool the oil, preventing it from overheating89.

16.4 Oil Properties and Types

•  Viscosity: The oil's resistance to flow. Viscosity decreases as temperature increases90909090.
• An oil's ability to maintain its properties over a range of temperatures is known as its
• Viscosity Index91.
•  Hydraulicing: On radial or inverted engines, oil can drain into the lower cylinders after
• shutdown. If the engine is started in this condition, the incompressible oil can cause severe
• damage (a 'hydraulic lock')92. These engines must be turned by hand (pulling the propeller
• through) before starting to check for and clear any fluid93.