Lubricating oil functions and lubricating system

Lubrication in a diesel engine means supplying a film of oil between surfaces that move relative to each other, so that metal-to-metal contact is avoided.

Functions of lubricating oil

1. Reduces friction and wear of pistons, rings, liners, bearings, cams and gears; this also reduces friction power loss.
2. Cooling: carries heat away from bearings and from the underside of piston crowns.
3. Sealing: forms a seal between piston rings and cylinder liner, preventing blow-by of gas.
4. Cleaning: washes away carbon, metal particles and dirt to the filters.
5. Corrosion protection of metal surfaces against acids formed by combustion.
6. Shock absorption / noise reduction in bearings and gears.

Sketch: forced-feed (pressure) lubrication system

   +--------------- Engine ---------------+
   | main brgs, crank pins, gudgeon pins, |
   | camshaft, rocker arms, cyl. walls    |
   +--------------------------------------+
        ^                       |  drains
        | oil gallery           v
   Oil cooler <- Fine filter  SUMP / oil tank
        ^              ^        |
        |              |   strainer
        +---- Relief --+-- PUMP <+
              valve      (gear)
       raw cooling water through cooler

Working

1. A gear pump driven by the engine draws oil from the sump (or a separate oil tank in dry-sump systems) through a coarse strainer.
2. The oil is pumped at about 2–4 bar through a fine filter and an oil cooler (cooled by water from the cooling system). A pressure relief valve returns excess oil to the sump.
3. Oil enters the main oil gallery and is distributed through drilled passages to the main bearings, then through holes in the crankshaft to the crank-pin bearings and up the connecting rod to the gudgeon pin.
4. Branches feed the camshaft bearings, valve gear and rocker arms. Cylinder walls are lubricated by oil splash from the crank or by separate cylinder lubricators in large engines.
5. Oil drains back by gravity to the sump; in large plants it is periodically purified in a centrifuge to remove water and sludge.
6. A pressure gauge and temperature gauge (with low-pressure alarm/trip) protect the engine.

Other lubrication methods are splash and mist (oil mixed with the fuel, used in small two-strokes) systems. A forced-feed system may be wet sump or dry sump; power plants use forced-feed systems.