Engine lubrication systems and SAE grades

(a) Lubrication

Functions: reduce friction and wear, cool the parts (piston underside, bearings), seal the piston rings, clean (carry dirt away to the filter) and protect against corrosion.

1. Splash system: a dipper on the connecting rod big end splashes oil from the sump onto the cylinder walls and bearings. It is simple and used in small engines (e.g. lawn mowers), but the oil supply varies with speed and oil level.

2. Pressure (wet-sump) system: a gear pump picks up oil from the sump, passes it through a filter and feeds it at 2 to 4 bar through the main gallery to the main bearings, big-end bearings (through drillings in the crankshaft), camshaft and valve gear. A relief valve limits the pressure, and the oil drains back to the sump.

 Sump -> strainer -> pump -> filter -> main gallery
                        |  (relief valve)   |-> crank bearings
                        +<------------------ |-> cam, valves

3. Dry-sump system: oil drains to a small collector; a scavenge pump moves it to a separate external tank, and a pressure pump feeds the engine. It is used in racing and sports cars and in aircraft, because the engine can be mounted lower and there is no oil starvation during hard cornering or on steep inclines.

(b) SAE grade and viscosity index

SAE viscosity grade: the Society of Automotive Engineers rates oil by viscosity. In 15W-40, "15W" (W for winter) shows that the oil behaves like an SAE 15 oil when cold (it can be cranked and pumped down to about -20 degrees C), while "40" shows that at 100 degrees C it behaves like an SAE 40 oil. It is a multigrade oil for use all year: thin when cold for easy starting, but thick enough when hot to protect the engine.

Viscosity index (VI): a number that shows how little the viscosity changes with temperature. A high VI means the oil thins less when hot. Multigrade oils use polymer additives to give a high VI.

Oil must also meet quality grades such as API SN (petrol) and CK-4 (diesel).