The oil pump is an internal combustion engine part that circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine. This lubricates the bearings, allows the use of higher-capacity fluid bearings and also assists in cooling the engine.
As well as its primary purpose for lubrication, pressurized oil is increasingly used as a hydraulic fluid to power small actuators. One of the first notable uses in this way was for hydraulic tappets in camshaft and valve actuation. Increasingly common recent uses may include the tensioner for a timing belt or variators for variable valve timing systems.
Pumps
The type of pump used varies. Gear pumps trochoid pumps and vane pumps are all commonly used. Plunger pumps have been used in the past, but these are now only used rarely, for small engines.
To avoid the need for priming, the pump is always mounted low-down, either submerged or around the level of the oil in the sump. A short pick-up pipe with a simple wire-mesh strainer reaches to the bottom of the sump.
Pump drive
For simplicity and reliability, mechanical pumps are used, driven by mechanical geartrains from the crankshaft. Reducing pump speed is beneficial and so it is usual to drive the pump from the cam (if this is mounted in the cylinder block) or distributor shaft, which turns at half engine speed. Placing the oil pump low-down uses a near-vertical drive shaft, driven by helical skew gears from the camshaft. Some engines, such as the Fiat Twin Cam engine of 1964, began as OHV engines with an oil pump driven from a conventional camshaft in the cylinder block. When the twin overhead cam engine was developed, the previous oil pump arrangement was retained and the camshaft became a shortened stub shaft. Even when the distributor position was moved from the previous block-mount to being mounted on the cylinder head camshafts, the oil pump drive remained in the same position, the unused distributor position now covered by a blanking plate. Small engines, or scooters may have internal gear pumps mounted directly on their crankshaft.
For reliability, it is rare to use an external drive mechanism, either a separate belt drive or external gears, although camshaft-driven pumps often rely on the same timing belt. Additional separate belts are sometimes used where dry sump pumps have been added to engines during tuning.
Electric oil pumps are not used, again for reliability. Some 'turbo timer' electric auxiliary oil pumps are sometimes fitted to turbocharged engines. These are a second oil pump that continues to run after the engine has stopped, providing cooling oil to the hot bearings of a turbocharger for some minutes, whilst it cools down. These are supplementary pumps and do not replace the main, mechanical, oil pump.
Electric pump as a main engine pump again will require big electric motors and it may be simply cheaper to drive directly from the engine. For eg BMW S65 engine's oil pump delivers ca. 45 LPM (Litres Per Minute) of oil at 5.5 bar pressure. This pump would require a significantly large motor to drive.
Lubricating system
The oiling system addresses the need to properly lubricate an engine when it's running. Properly lubricating an engine not only reduces friction between moving parts but is also the main method by which heat is removed from pistons, bearings, and shafts. Failing to properly lubricate an engine will result in engine failure. The oil pump forces the motor oil through the passages in the engine to properly distribute oil to different engine components. In a common oiling system, oil is drawn out of the oil sump (oil pan, in US English) through a wire mesh strainer that removes some of the larger pieces of debris from the oil. The flow made by the oil pump allows the oil to be distributed around the engine. In this system, oil flows through an oil filter and sometimes an oil cooler, before going through the engine's oil passages and being dispersed to lubricate pistons, rings, springs, valve stems, and more.
Oil pressure
The oil pressure generated in most engines should be about 10 psi per every 1000 revolutions per minute (rpm), peaking around 55-65 psi.
Local pressure (at the crankshaft journal and bearing) is far higher than the 50, 60 psi &c. set by the pump's relief valve, and will reach hundreds of psi. This higher pressure is developed by the relative speeds in feet per second (not RPM or journal size directly) of the crankshaft journal itself against the bearing, the bearing width (to the closest pressure leak), oil viscosity, and temperature, balanced against the bearing clearance (the leakage rate).