Marine diesel engine is a type of diesel engine used on ships. Marine diesel engines are the main power of civilian ships, small and medium-sized ships and conventional submarines. Marine diesel engine can be divided into main engine and auxiliary engine according to its function in ship. The main engine is used as the propulsion power of the ship, which is used to drive the generator, air compressor or water pump. The main engine is used as propulsion power of ships, auxiliary engines are used to drive generators, air compressors or water pumps.