From our insanely detailed guide:

Oil pump

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The earliest engines used a drip feed system for oil - dropping it into moving parts and then collecting it in a tray underneath. On ships there was a position as ‘oiler’ whose job was to fill up little pots of oil above moving components. From around 1900 onwards, internal combustion engines moved to a forced-feed system, where oil is pumped around (forced) to the various parts of the engine. All modern engines use a fully forced-feed lubrication system and the key to this is the oil pump.

The oil pump is the heart of the lubrication system. It sucks oil up from the oil pan and forces it around oilways in the engine, before the oil drops back into the sump and is recirculated.

Oil pumps are situated in the oil pan, or more usually at the front of the engine.  

Oil pickup pipe

The oil pump sucks oil up from the sump through a pipe - called a pickup pipe. The pipe’s nozzle sits below the surface of the oil, and it is covered by a guaze filter which prevents large particles from being sucked up into the pump.

If this filter screen was to be completely blocked, then the engine would not pick up any oil and the engine would at great risk of being destroyed. Therefore the pickup pipe may have bypass valve in case the screen is blocked. In this situation it’s better that the engine continues to receive oil and we take the chance of damage to the oil pump: an oil pump is considerably cheaper than an engine rebuild.

Oil pumps

Most oil pumps are driven directly by the crankshaft. The pump illustrated here sits over the nose of the crankshaft where its internal gear is driven directly.

All oil pumps are called positive displacement pumps - the amount of oil that leaves is the same as that enters. In other words, the pump moves oil from one side, to the other. This can be contrasted with a coolant pump, for example, which tries to move water but will not do so if the pressure is too great on one side.

As engine RPM increases, the pump turns faster and more oil is pumped. This is convenient because at higher engine speeds lubcrication and cooling requires a greater flow of oil.  

A note on oil pressure

It is important to note that oil pressure is not produced by the oil pump. The oil pressure is caused by restrictions to the flow of oil - through narrow passageways, jet nozzles, and tight bearing clearances. If we simply connected a pipe to the outlet side of the oil pump and allowed it to discharge back into the sump, then there would be no oil pressure - the oil could flow unrestricted.

Knowing this, it’s now clear why worn bearings (particularly main bearings) would lead to a loss of oil pressure: the clearance in these bearings is now larger, and the oil’s flow is less restricted.

Oil pump mechanisms

There are two main types of oil pump in use. Both use metal gears for longevity and the ability to pump against high pressures.

Rotor-type pumps

Rotor type pumps are the most common. Sometimes called gerotors, or trochoidal pumps. The pump has two gears - one inner gear, which is driven and one outer gear which rotates around the inner gear. The inner gear has one less tooth than the outer gear, leaving a space into which expands and sucks oil in to fill it. As the rotation continues, that space is slowly closed as the gear teeth mesh together once again, and the oil is forced out of the gap and out of the pump.

Gear-type pumps

A gear-type oil pump has two gears of the same size, which mesh against each other. One gear is driven, and interlocks with the undriven gear to mean that they both rotate at the same time. As the teeth move around, they carry oil in the gap, until meeting in the middle and meshing together once more - forcing the oil out.  

Pressure relief valve

Both these pump mechanisms will continue pumping oil up to extremely high pressures. In order to prevent damage to the engine, an oil pump includes a pressure-relief valve which will open when oil pressure becomes to high.

The pressure relief valve is on the output side of the pump. The oil on the output side pushes on a piston, or ball bearing. That piston is held closed by a calibrated spring and once the pressure reaches a determined level, the spring will flex and the piston will unseat, allowing oil to return either to the inlet side of the pump or directly down into the sump.  

Continue reading: Oil filters