The ship's low-speed diesel engine is the main engine of the ship and mainly provides power for the ship. According to the characteristics of the working cycle, low-speed machines can be divided into two-stroke low-speed machines and four-stroke low-speed machines.

(1) Two-stroke low-speed diesel engine

Two-stroke low-speed diesel engines are the leading products of marine low-speed diesel engines, and serve as the main engines of large ships. The improvement of the two-stroke low-speed machine from the ordinary model to the modern model is mainly reflected in the increase in power and reduction in unit fuel consumption, while meeting the requirements of international marine environmental protection. The substantial increase in power greatly meets the needs of building large and giant ships; the reduction in unit fuel consumption meets the human pursuit and desire for environmental protection and meets the requirements of environmentally-friendly engines.

(2) Four-stroke low-speed diesel engine

Four-stroke low-speed diesel engine is a kind of ship's low-speed diesel engine, which is rarely seen in the world. Its structure is the same as that of the two-stroke low-speed machine, and it is divided into two parts, the cylinder and the crankcase. The crankcase works the same way as a four-stroke machine, and the cylinder works the same way as a two-stroke machine. According to the size of the model, the number of cylinders is generally between 4 and 8. The stroke and unit fuel consumption are close to those of two-stroke low-speed machines with the same number of cylinders, which can also meet the requirements of international marine environmental protection. But the speed is higher, and the maximum design speed can reach 390 r / min.

Japan ’s marine diesel engine design manufacturers usually have technology patents for four-stroke low-speed diesel engines, and are often used in various types of ships made in Japan, such as various transport vessels, engineering vessels, and marine fishing vessels as the main engine.

Large low-speed diesel engines have always been the main engines of marine propulsion engines due to their high reliability and the ability to burn low-quality fuel. Secondly, the maintenance cycle of the low-speed machine can reach tens of thousands of hours, and the high-speed machine must be overhauled to more than 10,000 hours. Finally, because the structure of the low-speed machine is larger than the high-speed machine, the speed is relatively low, so the operation is relatively stable.

The advantages of low speed, life, wear, and maintenance are better than high speed. The advantage of high speed is its small size, which is more suitable for loading cars and other places with volume restrictions. The disadvantage is that maintenance is difficult and the life is not as good as low speed.