The difference between low-speed, medium-speed and high-speed marine diesel engine parts:

1. Low-speed diesel engines are mostly two-stroke engines, medium-speed diesel engines are mostly four-stroke engines, and high-speed diesel engines have both. The scavenging forms of marine two-stroke diesel engines include backflow scavenging, port-valve DC scavenging and opposed piston port scavenging. High-power medium and low-speed diesel engines widely use heavy oil as fuel, and high-speed diesel engines still use light diesel.

2. The low-speed diesel engine directly drives the propeller. In order to make the propeller have high propulsion efficiency, a lower speed is required. Medium and high-speed diesel engines drive propellers through gear reduction boxes. Gear boxes are generally equipped with a reverse mechanism to achieve propeller reversal, but low-speed diesel engines and some medium-speed diesel engines can reverse themselves. Medium and high-speed diesel engines also realize electric transmission through generator-motor-propeller.

Diesel engines have high thermal efficiency, good economy, easy starting, and great adaptability to various types of ships. After they came out, they were quickly used as ship propulsion power. By the 1950s, diesel engines had almost completely replaced steam engines in newly constructed ships. Marine diesel engines have become the main power of civil ships, small and medium-sized ships and conventional submarines (see Ship Power Plant). Marine diesel engines can be divided into main engines and auxiliary engines according to their roles in ships. The main engine is used as the propulsion power of the ship, and the auxiliary engine is used to drive the generator, air compressor or water pump.