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 the power consumption inside the engine is increased, and the working surfaces of the parts are quickly worn; the heat generated by friction may also cause the surface of some working parts to melt, causing the engine to malfunction. Therefore, in order to ensure the normal operation 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.