Chapter 16: 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.