The Piston Engine

Chapter 16: Engine Lubrication Systems

Technical General for Aviators — Capt. Pankaj Pahil

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.

 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.

 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.

 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.