The engine lubrication system is designed to deliver clean oil at the correcttemperature and pressure to every part of the engine. The oil is sucked out of the sumpinto the pumpp being the heart of the system，than forced through an oil filter andpressure feeded to the main bearings and to the oil pressure gauge. From the mainbearings，the oil passes through feed-holes into drilled passages in the crankshaft and on tothe big-end bearings of the connecting rod. The cylinder walls and piston-pin bearings arelubricated by oil fling dispersed by the rotating crankshaft. The excess is scraped off bythe lower ring in the piston. A bleed or tributary from the main supply passage feeds eachcamshaft bearing. Another bleed supplies the timing chain or gears on the camshaft drive.The excess oil then drains back to the sump，where the heat is dispersed to thesurrounding air.
A lubrication system has several important functions：
Engine oil - lubricant for moving parts in engine
Oil pan - reservoir or storage area for engine oil
Oil pump - forces oil throughout the inside of the engine
Pressure relief valve - limits maximum oil pump pressure
Oil filter - strains out impurities in the oil
Oil galleries - oil passages through the engine
Fig. 6. 3 shows a lubrication system consisting of the following:
With the engine running，the oil pump pulls engine oil out of the oil pan. Before oilenters the pump，a screen on the pickup tube removes large particles from the oil. Thepump then pushes the oil through the oil filter and oil galleries.
The oil filter cleans the oil, removing very small particles. The filtered oil then flowsto the camshaft, crankshaft，balancer shafts, lifters, rocker arms，and other movingparts.
When oil leaks out of the engine bearings, it sprays on the outside of internal engineparts. For example, when oil leaks out of the connecting rod bearings，it sprays on thecylinder walls. This lubricates the piston rings，pistons，wrist pins, and cylinders. Oilfinally drains back into the oil pan for recirculation.
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