When the engine is running, the friction surfaces (such as crankshaft journals and bearings, camshaft journals and bearings, piston rings and cylinder walls, timing gear pairs, etc.) make relative movement at high speeds,and the friction between metal surfaces not only increase the power consumption, but also quickly wear the working surfaces of the moving parts. The heat generated by friction may also cause the surface of some working parts to melt, thus results in engine malfunction. Therefore, in order to ensure the normal performance of the engine,the surface of the relatively moving parts in the engine must be lubricated, that is, the friction surface is covered with a layer of lubricant (oil or grease), and a thin oil film is separated between the metal surfaces to reduce Friction resistance, reduce power loss, reduce wear, and extend engine life.