What's with the oil in the piston of a car engine?

It is normal to have oil in the piston of an engine. Because pistons are lubricated by splashes of oil. Specific process is: there is a gap between piston and cylinder, piston ring below there is a small hole, when the oil through the crank splash effect, and the crankshaft in the oil, the oil to the piston inside, again by holes flow into the lubrication between the piston and cylinder, scrape off the excess oil and oil ring back into the engine, repeated movement for lubrication.

Oil ring is mainly scraping oil, cloth oil and auxiliary sealing function. The oil ring is used to scrape the excess oil on the cylinder wall, and spread a layer of uniform oil film on the cylinder wall, which can prevent the oil string into, and can reduce the wear and friction resistance of the piston and cylinder. Oil ring is divided into ordinary oil ring and combined oil ring. The ordinary oil ring is generally made of cast iron, the middle of the outer circle is cut into a groove, and the bottom of the groove is processed with many oil holes. The composite oil ring is composed of an oil scraper and two elastic lining rings. The axial lining ring is clamped between the second and third oil scraper.