Oil pressure (internal combustion engine)
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Oil pressure is an important factor in the longevity of most internal combustion engines. With a forced lubrication system (invented by Frederick Lanchester), oil is picked up by an oil pump and forced through oil galleries (drillings) into the most highly-stressed bearings, such as the main bearings, big ends and camshaft bearings. Other components such as cam lobes and cylinder walls are lubricated by oil jets.
Sufficient oil pressure ensures that the metal of the rotating shaft (journal) and the bearing shell can never touch, and wear is therefore confined to initial start-up and shutdown. The oil pressure, combined with the rotation of the shaft, also hydrodynamically centres the journal in its shell.
As bearing clearance increases due to wear and the oil pump declines in efficiency with age, oil pressure falls. Eventually it may be insufficient to protect the bearings, especially if the engine is "lugged" at low rpm. Some engines include an oil pressure gauge which shows the pressure at any given time, while others just employ a warning light which shows when the pressure is insufficient. The sensor for either type is usually mounted near the oil filter of the engine, just after the oil pump.
Oil pressure is higher when the engine is cold due to the increased viscosity of the oil, and also increases with engine speed. Oil pressure is lowest under hot idling conditions, and the minimum pressure allowed by the manufacturer's tolerances is usually given at this point.
Excessive oil pressure may indicate a blocked filter, blocked oil gallery or the wrong grade of oil.
