1. There is oil leakage, which can be checked by checking the engine appearance to see if there is oil leakage.
2. It may be that the piston ring is in the right position, or it is broken (especially the oil ring), which leads to poor sealing of the cylinder, so that the oil enters the cylinder and burns.
3. either the piston ring (especially the oil ring) is seriously worn, or the wear between the piston and the cylinder liner is serious, resulting in poor seal of the cylinder, so that the engine oil enters the cylinder and is burned.
4. The cylinder gasket is damaged, which causes the oil to enter the cylinder and burn.
5. The oil cooler is broken, which makes the oil enter the cooling water. You can know if there is oil in the water tank.