Marine engines and their classification

Ship engines can be divided into main engine and auxiliary engine according to their role in the ship. The main engine drives the propeller, and the auxiliary engine drives the generator. According to the engine speed, it is generally divided into low-speed diesel engine, medium-speed diesel engine and high-speed diesel engine. According to the engine combustion cycle, it can be divided into two-stroke and four-stroke diesel engines. The crankshaft of a two-stroke diesel engine rotates once per working cycle. The intake process is included in the exhaust process, and the intake can also help exhaust at the same time, which is called scavenging. 

Every time a four-stroke diesel engine completes a working cycle, the crankshaft rotates for two revolutions, and only the expansion stroke does external work in each working cycle.

Low-speed diesel engines have low rotational speed, large cylinder diameter, long stroke, and large output power. Generally, in-line cylinders, two-stroke, and cross-head structures are used. 

The piston rod and the connecting rod are connected by a cross head sliding along the guide plate, the piston is connected with the connecting rod through the cross head through the piston rod, and the guiding function is assumed by the cross head. When the low-speed engine is used as the main engine, it can directly drive the propeller, and can also reverse the rotation and reduce the reduction gear box and other equipment, so as to reduce the cost. Major large merchant ships use low-speed engines as their main engines.
The low-speed engine brands are very concentrated. The world's three major low-speed engine manufacturers are MAN, CSSC Winterthur (acquiring Wärtsilä's two-stroke business), and Japan's Mitsubishi Heavy Industries.

Electronic control technology is also widely used in marine diesel engines. Compared with traditional camshaft control fuel injection, electronic fuel injection can make the main engine combustion control more accurate, more efficient, lower carbon, and can effectively control and meet emission standards. The biggest difference between the electronic control host and the traditional host is that the camshaft is cancelled, and the actions of the fuel pump, exhaust valve and cylinder lubrication are all controlled by the electronic system. 

Medium-speed engines are generally used as auxiliary engines to drive generators; however, they are also commonly used as main engines in inland shipping ships, offshore shipping ships, engineering dredging ships, and official ships. At this time, due to their high rotational speed, it is necessary to drive the propeller through a reduction gearbox, or directly with variable pitch propellers. 

In recent years, LNG (liquefied natural gas) has gradually been recognized by the industry as a clean energy source. However, the LNG infrastructure is not perfect, the ship's own storage capacity is limited, and the cruising range of ships that use LNG alone is limited. In this context, the four-stroke dual-fuel engine came into being, gradually applied to ships.

The cost of burning liquefied natural gas is lower. On the other hand, the global natural gas reserves are very rich. The dual-fuel power engine can work under a single fuel state and can be easily converted between dual fuels. It has strong flexibility and is conducive to ships use traditional fuel to improve endurance. 

During the operation of dual-fuel powered ships, the main engine mainly burns LNG, and diesel is used as the pilot fuel, which only accounts for 10-15% of the fuel consumption of existing ships using diesel engines. Real-time control of pilot diesel quantity, natural gas injection quantity and the number of injection cylinders, so that the main engine can achieve the lowest fuel consumption. In gas mode, it can meet the IMO Tier 3 emission requirements, greatly reducing emissions.