Oil pumps may be driven from the camshaft or the crankshaft.
In a rotor-type oil pump, an inner rotor drives an outer one. As they turn, the volume between them increases. This larger volume lowers the pressure at the pump inlet.
Outside atmospheric pressure is then higher. This forces oil into the pump, and it fills the spaces between the rotor lobes.
As the lobes of the inner rotor move into the spaces in the outer rotor, oil is squeezed out through the outlet.
The crescent pump uses a similar principle. It is mounted on the front of the cylinder block.
The inner gear is on the end of the crankshaft which then drives the pump directly. An external toothed gear meshes with this inner one. Some gear teeth are meshed but others are separated by the crescent-shaped part of the pump housing.
The increasing volume between gear teeth causes pressure to fall. Oil is then taken through the intake port, and carried around between the gears and crescent, then discharged to the outlet port.
Similarly in a geared oil pump, the driving gear meshes with a second gear. As both gears turn, their teeth separate, creating a low pressure area. Higher atmospheric pressure outside forces oil up into the inlet. The spaces between the teeth fill with oil. The gears rotate, and carry oil around the chamber. The teeth mesh again, and oil is forced from the outlet toward the oil filter.
A normal pump is capable of delivering more oil than an engine needs. It’s a safety measure to ensure the engine is never starved for oil. As the pump rotates, and engine speed increases, the volume of oil delivered also increases. The fixed clearances between the moving parts of the engine prevent oil escaping back to the sump, and pressure builds up in the system.
An oil pressure relief valve stops excess pressure developing. It’s like a controlled leak, releasing just enough oil back to the sump to regulate the pressure of the whole system.
There are 2 basic oil-filtering systems - full-flow, and by-pass. The full-flow type filters all of the oil before delivering it to the engine. The by-pass type only filters some of the oil.
The full-flow type is the more common. Its filter uses pleated filtering paper in a metal housing, to collect harmful particles.
Normally all oil goes through the filter before it gets to the engine, but if the filter clogs up, it can starve an engine of oil.
As a safety measure, full-flow filters have a bypass valve. If the filter clogs, this valve opens and directs unfiltered oil to the engine. Dirty oil is better than none at all.
Most oil-filters on diesel engines are larger than those on similar gasoline engines, and some diesel engines have 2 oil filters.
Diesel engines produce more carbon particles than gasoline engines, so the oil filter can have a full-flow element to trap larger impurities, and a bypass element to collect sludge and carbon soot.
In a by-pass system, the bypass element filters only some of the oil from the pump by tapping an oil line into an oil passage. It collects finer particles than a full-flow filter. After this oil is filtered, it goes back to the sump.
Pistons, rings and pins are lubricated by oil thrown onto the cylinder walls from the connecting rod bearings.
Some connecting rods have oil spurt holes. These holes are positioned to receive oil from similar holes in the crankshaft. Oil can then spurt out at the point in the engine cycle when the largest area of cylinder wall is exposed. It lubricates the walls and gudgeon pin, and also cools the underside of the piston.
Oil feeds to the cylinder head, and through a gallery to the camshaft bearings and valve-train. As well as lubricating these moving parts, it also gathers heat from the engine so its temperature keeps rising.
Finally it drains back to the sump to cool, and start again.
Automotive services Oil indicators
If a lubrication system fails it’s serious, so it’s crucial to know it’s working. If oil pressure falls too low, a pressure sensor in a gallery can light up a warning light, or register on a gauge.
Low oil pressure can mean a lack of oil. It may have leaked away, or it may have been burned.
This can be caused by worn piston rings which let oil into the combustion chamber.
Some engines even use an automatic cut-out that turns off the engine if oil pressure falls too low.
Too little oil in the engine is a problem but so is too much. The crankshaft can whip it into foam, and cause leaks by flooding the seals..
Of course, the simplest indicator of oil level is still the dip stick.
Automotive services Oil cooler
Engines which operate under severe conditions may use an oil cooler to cool the oil in the engine.
In diesel engines, the oil cooler and oil filter are often on the same mounting , on the cylinder block.
The oil cooler is a heat exchanger. It transfers heat from the oil to coolant from the cooling system. Coolant circulates through tubes in the cooler, and oil fed from the lubrication system surrounds the tubes. As the coolant circulates, heat is removed from the oil.
In another design, the oil cooler is mounted in the airstream at the front of the vehicle. This type of oil cooler uses the flow of air passing across its fins to cool the air circulating through it. It is called an oil-to air heat exchanger.
