Abstract: The engine used for ship propulsion is called the main propulsion engine (also known as the “main engine”), which plays a decisive role in the performance of the entire marine power plant. The types of marine power plants are usually divided according to different main engines. The main engines of modern ships mainly include diesel engines, steam turbines, gas turbines and nuclear power plants. At present, most civilian ships and some medium and small military ships use diesel engines; large and medium-sized ships mostly use steam turbines or gas turbines; combined power plants can meet the needs of surface ships under different navigation conditions; nuclear power plants can significantly improve the endurance and self-sufficiency of ships, but they need to use shielding devices, are heavy and have complex structures. They are suitable for large aircraft carriers, large and medium-sized submarines, icebreakers and some large civilian ships. Medium and small submarines can use diesel engines to drive generators to generate electricity and drive the submarines forward by electric propulsion. By studying marine power plants, it is helpful to quickly grasp their types, structures, technical characteristics, use and management methods, thereby improving their actual application effects during ship navigation and improving the overall power performance of ships.
Keywords: diesel engine; gas turbine; steam turbine; combined power unit; nuclear power; aircraft carrier; electric propulsion

In the early 19th century, steam engines were successfully used in the inland ship “Clermont” as the main propulsion power for the entire ship, which marked the formation of the concept of “power unit” for ships. The original meaning of power unit refers to a set of machinery, equipment and systems that replace human power or wind power to provide propulsion power for various ships. Over the years, with the continuous development and optimization of ship technology, the performance of related power units has also been gradually improved.

1 Overview of marine power units
In marine power units, the engine that provides the power source for ships to sail is called the main propulsion engine, also known as the main engine of the ship. The main engine converts heat energy from various sources into mechanical work, thereby providing energy required for navigation for various ships. Power units are divided into several categories according to specific types, mainly including diesel engines, steam turbines, gas turbines, combined power units, and nuclear power units.

The ship’s main engine should strive to be safe and reliable, have sufficient vitality, be easy to use, flexible, light weight, small size, low cost, low fuel consumption, easy maintenance and long life, have the ability to reverse, and can run stably at low speed or cruising speed, so as to ensure that the various performances of the ship are maximized. The relative importance of these characteristics varies with the different tasks of the ship, and reliability is the most important.

As mentioned above, the ship’s main engine is the core equipment in the power plant. The selection of the main engine type determines the structural composition and performance characteristics of the entire marine power plant in general. Therefore, the classification of marine power plants is mostly based on the type of main engine. The working condition of the main engine directly affects the normal navigation and safety of the entire ship, and special attention should be paid when selecting, designing and manufacturing.

2 Technical characteristics and development overview of marine diesel engines

2.1 Overview of marine diesel engines

Diesel engines are a common type of internal combustion engine. Since they do not need to be equipped with other equipment such as boilers, the system redundancy is reduced. They are currently widely used on various types of ships. However, due to the reciprocating motion of diesel engines, wear, vibration and noise are relatively large. In particular, the increase in diesel engine power is limited by processing equipment, technology, materials, volume and weight, so it is difficult to further increase the power of a single engine.

Among diesel engines, low-speed diesel engines have the lowest fuel consumption rate and can burn inferior heavy diesel, so the fuel cost is low. Due to the low speed and small wear of low-speed diesel engines, the service life is long and the maintenance cost is low. However, the weight and size of low-speed diesel engines are large, and they occupy more cabin space and displacement, which is their disadvantage. This type of engine is mostly used as the main engine of large ships.

High-speed diesel engines are light in weight and small in size, and they occupy less cabin space and displacement. However, high-speed diesel engines have a high fuel consumption rate and need to use high-quality light diesel, so the fuel cost is relatively higher. At the same time, due to the high speed of high-speed diesel engines, the machine wears more, the service life is short, the maintenance cost is high, and the noise is also high. This type of engine is mostly used as the main engine of small ships or the auxiliary engine of large and medium-sized ships. The performance of medium-speed diesel engines is between the above two, and they are generally used in medium-sized ships. However, in recent years, as medium-speed diesel engines can also burn heavy oil, and the fuel consumption rate is close to that of low-speed diesel engines, with the development of reduction gearbox technology, some medium-speed high-power diesel engines have also been used in large ships.

2.2 Main technical features of marine diesel engines

In general, the technical advantages of diesel engines are mainly in the following aspects.

1) Good economy. Diesel engines have high economy in a wide range of operating conditions. At the same time, low-speed diesel engines can also burn heavy oil, which significantly reduces fuel costs. The fuel consumption rate of the ship’s main engine is an important factor in determining the economic efficiency of ship operations. Under the current energy situation, the high economy of diesel engines ensures its good application prospects.

2) Wide power range. The power and speed coverage of marine diesel engines is wide, and many models can be provided for various types of ships to choose from. Low-speed diesel engines are the most powerful models among diesel engines. Their technological development is representative. Their economic efficiency, reliability, and maneuverability have been significantly improved in recent years.

3) Good maneuverability. The diesel engine starts quickly, is easy to operate, and has sensitive reversing. The pre-start work can be completed in about 10 minutes. The transition time from cold start to full load operation of the main engine is generally no more than 10 minutes, and no more than 3 to 4 minutes in an emergency. The main engine reversing can generally be completed within a few seconds. The maneuverability of the main engine is an important technical performance indicator, which is especially important for surface ships. It will directly affect the comprehensive response capability and technical performance of the entire ship. In today’s era, high-performance weapons represented by missiles have been fully developed and pose a major threat to surface ships. Therefore, shortening the preparation time, quickly engaging in combat, and leaving the danger zone in time are of great significance to surface ships.

4) Low air consumption, small space occupied by the inlet and exhaust ducts, easier to arrange, and better independent working ability and impact resistance.

5) Low-speed diesel engines can directly drive propellers, and by adopting a lower speed, cavitation of the propeller can be avoided, thereby improving the propulsion efficiency of the propeller. This feature not only improves the overall efficiency of energy conversion, but also does not require a reduction device, simplifies the transmission equipment, and saves construction investment and maintenance costs during shipbuilding.

6) Strong environmental adaptability. Diesel engines can continue to work under the influence of continuously fluctuating high back pressure and large vacuum, and the power attenuation is not significant. In addition, diesel engines can also be made into low-magnetic units to meet the special requirements of surface ships such as minesweepers.

7) Medium and small diesel engines are light in weight, occupy less space, and have fewer auxiliary equipment. One of the main technical requirements for marine power units is lightweight to reduce the displacement occupied by the power unit, which can increase the net load of the ship or improve the speed and endurance of the ship. For medium and small ships, the engine room space is relatively narrow. In addition to requiring the power unit to be light in weight, it is also particularly required to be compact in size and small in volume. The weight and volume of the power unit itself depend to a large extent on the main engine and its required auxiliary equipment. With the development of high-pressure technology, the power of the diesel engine has been significantly improved while the weight and size of the diesel engine have not changed much, and the unit weight of the supercharged diesel engine has been further reduced. The diesel engine does not need to be equipped with large auxiliary equipment, and the required auxiliary equipment is also less. The advantages of medium and high-speed diesel engines in this regard are particularly prominent. This is also an important reason why medium and small ships generally use diesel engines.

At present, the technical disadvantages of diesel engines are mainly in the following aspects.

1) The single power of medium and high-speed diesel engines is relatively small.

2) Although the power of low-speed diesel engines is relatively large, as the power of the whole machine increases, the volume and weight of this type of unit will increase rapidly at a certain ratio. Therefore, there will be certain difficulties in designing and manufacturing low-speed diesel engines with higher power, and correspondingly, higher requirements are put forward for the processing, assembly and transportation of parts. In the process of reducing the mechanical and thermal loads of the unit materials, certain difficulties will also be encountered. The high height of the unit also limits its application on large surface ships such as aircraft carriers.

3) Unlike rotating machinery such as steam turbines and gas turbines, the pistons and other parts of diesel engines continue to reciprocate rather than simply rotate, so periodic disturbance forces will be generated. Therefore, the diesel engine not only has large vibration and noise, but also has serious friction and wear of the parts, and has strong low-frequency line spectrum vibration noise, which is very unfavorable for the stealth and anti-stealth of surface ships.

4) The minimum stable speed of the diesel engine is high, resulting in a relatively small stable working area of ​​the unit.

2.3 Overview of the technical development of marine diesel engines

As described in 2.2 of this article, the important advantages of diesel engines are high thermal efficiency, low fuel consumption rate, good overall economy, large power range, wide applicability, less ancillary equipment and light total weight. The main defects of diesel engines are noise, vibration and large friction losses. Large civilian ships mostly use low-speed diesel engines as main engines. This type of engine has high reliability, low maintenance costs, and can directly drive propellers. Medium and small military ships mostly use medium and high-speed diesel engines as main engines to reduce the size and weight of the unit.

In recent years, with the increase in ship tonnage, in order to increase the power of diesel engines, low-speed diesel engines are developing in the direction of increasing cylinder diameter and increasing supercharging, which can meet the requirements of high-power ship propulsion and can drive propellers through speed reduction devices. Especially in various types of inland ships, diesel engines have an absolute advantage and have almost become the only power type.

In recent years, how to save energy has become a major research topic at home and abroad. At present, relevant research work is being carried out to achieve a higher level of thermal efficiency of marine diesel engines. In addition to improving the combustion system, injection system, and supercharging system, and reducing friction loss and air leakage loss, research is also being conducted to reduce cooling loss, such as making full use of exhaust gas energy and installing a power turbine after the exhaust gas turbocharger to make full use of the energy of the exhaust gas. At present, there is still a lot of research and development work to be carried out on marine diesel engines.

In recent years, the technology of marine diesel engines has developed rapidly, mainly reflected in the following aspects.

1) High-power diesel engines generally use high-supercharging technology, and gradually improve the performance of units under low working conditions.

2) Adopt high-reliability modular design and manufacturing technology.

3) Medium and low-speed diesel engines use related technologies to fully burn heavy oil.

4) Adopt “intelligent” electronic control technology and high-pressure common rail fuel system technology, as well as low-emission and other related technologies.

In the past for a long period of time, civilian ships of various purposes generally used diesel engines as power units, while large ships mostly used steam turbines. In recent years, with the improvement of diesel engine heavy oil burning technology, the power of low-speed diesel engines has been significantly improved. Even on large ships, diesel engines have a tendency to gradually replace steam turbines. On inland ships, due to the limitations of objective conditions such as channel depth and ship tonnage, most inland ships use medium- and high-speed diesel engines as main engines.

3 Technical characteristics and development overview of marine steam turbines

3.1 Overview of marine steam turbines

Steam turbines are a type of thermal turbine machinery that uses steam expansion to convert thermal energy into mechanical energy. Among them, boilers, steam turbine bodies, condensers and feed pumps are relatively important equipment.

Like steam engines, steam turbines are power devices that convert steam thermal energy into mechanical work. The difference between the two is that in steam turbines, the thermal energy of steam is converted into steam kinetic energy, and this part of kinetic energy is then converted into mechanical work and transmitted to the turbine shaft. In terms of its working process, the high-pressure steam from the boiler enters the fixed nozzle, and the steam expands in the nozzle. When expanding, the pressure of the steam decreases, and the flow rate of the steam increases accordingly. The high-speed steam flow impacts the blades installed on the runner, causing the runner to rotate.

3.2 Main technical features of marine steam turbines

Generally speaking, steam turbines have the following technical advantages.

1) As the thermal engine with the highest single-unit power, it can effectively meet the power requirements of large surface ships.

2) It has high reliability and long service life, and its effective service life can reach more than 100,000 hours, and the operation, maintenance and maintenance process is relatively simple.

3) The unit has low vibration, friction and noise, which can provide a quieter and more comfortable environment for the ship’s personnel.

4) It has strong fuel adaptability and can use inferior fuel, which correspondingly improves economic performance.

But at the same time, steam turbines also have the following disadvantages.

1) The energy conversion process is complex and the economy is poor. In the process of energy transfer, heat energy will be lost in boilers, pipelines, valves, pumps and other equipment, especially in the condenser, so the thermal efficiency of the unit is relatively low. For steam turbines using simple cycles, their economy is poor, not as good as diesel engines or gas turbines. The main reason is that the initial temperature of the working fluid is low, and a large amount of heat energy will be taken away by the cooling water of the condenser, so the cycle efficiency is lower than that of the other two types of main engines.

2) The system composition is complex. The steam turbine uses steam as the working fluid, and must be equipped with boilers, condensers, pumps, and other auxiliary devices, or equipped with nuclear reactors and related systems to obtain high-temperature steam. Therefore, the weight index of the steam turbine is greater than that of medium-speed diesel engines, high-speed diesel engines and gas turbines. And affected by the steam preparation process, the maneuverability of the steam turbine is also not as good as the above units.

3) Due to the high speed, the steam turbine needs to be equipped with a reduction device, which further increases the weight of the unit, makes the system composition more complicated, increases the design and manufacturing costs, and reduces the system reliability.

3.3 Overview of the technical development of marine steam turbines

Since steam turbines rotate, they work smoothly, have low vibration and noise, and produce less friction and wear, and have a long service life, making them particularly suitable for passenger ships. Steam turbines are easy to maintain and have high reliability. They can operate under full load conditions for a long time, have strong overload capacity, and are more environmentally adaptable. Steam turbines have high power, can burn heavy oil or liquefied natural gas, and are smaller than steam engines in terms of weight and volume. However, the energy conversion process of steam turbines is more complicated, with low thermal efficiency, high fuel consumption rate, and poor economy. Currently, they are mostly used in large-tonnage oil tankers, container ships, and liquefied natural gas ships.

Since steam turbines can only rotate in one direction. In order to obtain the power for reverse rotation, a reverse stage is usually installed on the shaft of the low-pressure cylinder of the forward steam turbine. The reverse stage usually has no more than three rows of rotating blades, or may have only two rows, and the power it generates is about 40% of the forward power. During normal operation of the steam turbine, the reverse stage is in reverse state, so it is generally installed at the low-pressure end of the low-pressure steam turbine. The steam density of this part is low, so the gas resistance loss generated by the reverse steam turbine is also low.

In 1896, the United Kingdom successfully used steam turbines as ship main engines, and the trial speed could reach 34.5 kn (knots). Since then, steam turbines have been widely used on high-power ships. Early steam turbines were used directly to drive propellers, and no reduction gear was used. In order to make the propeller work at an ideal speed, a reduction gear was added to the steam turbine so that both the steam turbine and the propeller could operate at their respective optimal speeds. By 1916, almost all marine steam turbines used reduction gears, and the reduction ratio was increased from the initial 1:20 to more than 1:80. After adopting the reduction device, the steam turbine can run at a higher speed, the efficiency is significantly improved, the size of the machine body is correspondingly reduced, the whole device is more compact, the total weight is greatly reduced, and the working efficiency of the propeller is greatly improved, making the steam turbine an ideal high-power marine power device. Many large passenger ships, supertankers and high-speed container ships use steam turbines.

For a long time, due to its significant advantage in output power, steam turbines have certain application prospects in various large ships, especially in the field of large surface ships. However, due to the late start of my country’s steam turbine manufacturing industry, the proportion of ships using steam turbines as main engines is not high. With the improvement of my country’s shipbuilding industry system, it is still expected to be fully developed. At present, there are two main trends in the development of marine steam turbines at this stage: one is to improve the thermal efficiency of the system, by increasing the initial parameters of steam and adopting complex cycles, thereby improving the efficiency of the main engine and auxiliary engines; the other is to use lower steam parameters and increase the steam flow rate, so that the turbine body and boiler can adopt a simpler structural system, thereby simplifying the management process and enhancing the reliability of the device.

4 Technical characteristics and development overview of marine gas turbines

4.1 Overview of marine gas turbines

Since their introduction, steam turbines and diesel engines have been widely used. As described in 2.2 and 3.2 of this article, diesel engines are a type of internal combustion engine, where the fuel burns inside the cylinder, and have the advantage of good maneuverability; steam turbines are a type of thermal turbine machinery, and their main advantage is that they have a large single-unit power. Gas turbines combine the advantages of both, and are a type of thermal engine that was formally developed in the mid-20th century after the two.

Like steam turbines, gas turbines are also a type of thermal turbine machinery, mainly composed of three parts: compressor, combustion chamber and turbine. Among them, the turbine mainly includes a supercharger turbine and a power turbine. The supercharger turbine is coaxial with the compressor, and the power turbine drives the propeller through the shaft system, which is usually also called a dual-shaft gas turbine. The compressor, combustion chamber and supercharger turbine together constitute the gas generator.

4.2 Main technical features of marine gas turbines

During the development process, gas turbines were first widely used in the field of aviation propulsion and fully replaced piston engines. Since 1947, gas turbines have also been used in the field of surface ships, and have made great progress in the following decades. They have gradually become one of the main power units of surface ships and have been highly valued by navies around the world. Their technical advantages are mainly as follows.

1) Good maneuverability, excellent starting and acceleration performance. The gas turbine starts from the cold state and only takes 2 to 3 minutes to reach full load conditions. Once the enemy is discovered, the ship can respond quickly and quickly engage in combat, improving the maneuverability of combat and effectively shortening the preparation time. The above advantages are of great significance to surface ships.

2) The gas turbine is light in weight and small in size, can be made into a box body, and has a high single-unit power.

3) There are few accessories, and most of them are installed on the chassis, so the unit has strong vitality.

4) The degree of automation is high, and fewer staff are required.

5) The vibration amplitude of the unit is small, which can effectively improve the working environment of the ship’s personnel.

6) It is easy to repair, simple to manage, has a small maintenance workload, and is easy to realize automatic control.

Although gas turbines have outstanding technical advantages, they also have the following shortcomings.

1) The economy of gas turbines is not as good as that of diesel engines, especially when they deviate from the rated operating conditions, the fuel consumption rate of gas turbines will increase rapidly. Taking the WR-21 gas turbine as an example, its fuel consumption rate at rated conditions is similar to that of high-speed diesel engines, but when operating under low-load conditions, the fuel consumption rate of this type of gas turbine will increase rapidly. It is precisely because of the existence of the above problems that the application of gas turbines in civilian ships is limited.

2) Gas turbines cannot be reversed directly, and need to be equipped with a reverse transmission device or a pitch-adjustable propeller, which makes the power unit structure more complicated and increases the system cost.

3) The gas turbine has a large cross-sectional area for inlet and exhaust ducts, which affects the overall layout of the deck and cabin space of the surface ship.

4) The exhaust temperature is high and the heat radiation is strong, so its thermal signal characteristics are also strong, which affects the concealment of the entire ship.

5) It is sensitive to environmental conditions such as temperature, which easily affects the thermal efficiency of the unit.

6) The unit life is short. Since the combustion chamber and turbine blades of the gas turbine are continuously working under high temperature and high pressure conditions. At the same time, the sea air inhaled by the gas turbine contains a certain amount of salt. Under the action of substances such as sodium and vanadium, the turbine blades and nozzles may be corroded in a short time. Although high-quality alloy materials are usually selected, the service life of marine gas turbines is still short.

4.3 Overview of the technical development of marine gas turbines

Compared with steam turbines, gas turbines are small in size, light in weight, low in fuel consumption, good in starting and acceleration, simple in daily operation and maintenance, and convenient for remote centralized control. Compared with diesel engines, although gas turbines have relatively low thermal efficiency, they have higher power per unit, simpler structure, fewer parts, lighter weight, smaller size, and gradually improved reliability. Therefore, their application areas have been gradually expanded in recent years.

In recent years, some high-speed civilian ships have also begun to use gas turbines as the main propulsion power. The main ships of some countries all use gas turbines as propulsion power. With the growth of ship power demand, there are also plans to use gas turbines as power stations. The research and development of marine gas turbines in my country has also made significant progress and has broad development prospects.

At this stage, two different types of gas turbines are mainly developed: aviation-derived and industrial. Aviation-derived types have the characteristics of simple structure, light weight, and easy control, but they must use high-quality fuel. Industrial gas turbines are another type of model, which has a longer service life and can use properly treated heavy oil. Industrial units generally use regenerators. Fuel combustion requires a large amount of air and produces a large amount of exhaust gas, which makes air supply and flue gas emissions important issues.

Another reason for the rapid development of marine gas turbines is that they inherit and utilize the existing technical foundation of aviation gas turbines and industrial gas turbines. Especially for the former, aviation engines have always been the forerunners of gas turbine technology development. The principle of gas turbines has been known to people for a long time, but for a long time, compared with steam turbines, the application of gas turbines in industry has been relatively slow. The working fluid of steam turbines can condense into water, so the power consumed by the feed water pump is not large. However, the working fluid of gas turbines is air and gas that cannot condense into water, so the compressor needs to consume a considerable amount of energy to achieve the compression process. Therefore, only when the cycle temperature, compressor and turbine efficiency are high, can the gas turbine have a high cycle efficiency and provide a large useful power. It is the technical difficulties in high-temperature materials, blade cooling technology, and aerodynamic performance of compressors that restrict the development and application of gas turbines.

Compared with industrial gas turbines, marine gas turbines have more stringent requirements on the weight and size of the unit. However, this requirement is relatively loose compared with that of aviation gas turbines. On the contrary, the focus, or the main contradiction, of the aerodynamic design of marine gas turbines is often high component efficiency and stable variable operating characteristics. At the same time, in order to shorten the development cycle, it is necessary to reduce the debugging workload. When designing the main components of marine gas turbines, it is necessary to pay full attention to the requirements of mature technology, stable solutions, simple structure, and convenient manufacturing. Unlike aviation gas turbines, the cruising power of marine gas turbines is significantly lower than its maximum power. Although this particularity can be solved by adopting a combined power unit of a cruising unit and an acceleration unit, it still puts forward certain requirements for the economy of marine gas turbines in a wide range of load conditions.

At present, aviation-derived gas turbines have generally become the power units of military ships. Industrial gas turbines are more suitable for civilian ships, which have lower requirements for the weight and size of the power unit. If the ship needs to reverse, variable pitch propellers and electric propulsion can be used. In addition, closed-cycle gas turbines have high efficiency, but are still in the research stage. In ships, gas turbines are often put into use with combined power units.

For now, marine gas turbines have always been developing around increasing power, improving efficiency, and reducing size and weight. Its future development direction is mainly as follows.

1) Continue to develop simple cycles with higher initial parameters, continuously increase the initial temperature of the gas, and increase the pressure ratio accordingly, while adopting more efficient cooling technology. By adopting advanced cooling technology, the initial temperature of the gas can be increased by about 25 ℃ on average each year. In recent years, heat-resistant high-strength materials have also been continuously developed. By adopting high-temperature materials, the initial temperature of the gas can be increased by about 10 ℃ on average each year.

2) Continue to develop complex cycles and make full use of the exhaust heat of gas turbines to improve the overall efficiency of the unit. To this end, heat recovery cycles and gas-steam combined cycles can be used.

3) Further improve the performance of the main components of the gas turbine and improve the overall efficiency of the unit.

5 Comparison of technical parameters of marine main engines

Output power is a factor that large surface ships need to focus on when selecting main engines. Generally speaking, there are two main indicators that determine the actual output power of thermal engines: the flow rate of the working fluid and the specific enthalpy drop of the working fluid per unit flow rate.

Under the premise of similar unit size, gas turbines are significantly better than diesel engines in terms of output power. The main reason for the above phenomenon is the continuity of the working fluid flow of the gas turbine itself. The combustion process inside the gas turbine is in a continuous state, while the combustion process inside the diesel engine is intermittent. In order to avoid high-temperature failure and other phenomena, the peak temperature of the working fluid inside the gas turbine is usually lower than that of the diesel engine. Moreover, the compression effect of the axial compressor used in gas turbines is usually not as good as the piston mechanism of diesel engines. Therefore, in terms of the specific enthalpy drop of the working fluid per unit flow rate, gas turbines do not have an advantage. However, as mentioned above, since the working fluid inside the gas turbine is in a continuous flow state and there is no reciprocating throughput phenomenon, the working fluid has a clear advantage in terms of flow rate. In general, under the premise of similar structural size and weight, the output power of gas turbines is usually higher than that of diesel engines.

Compared with steam turbines, which are also thermal turbine machinery, the working fluid inside gas turbines is also in a continuous flow state. Although the temperature of the working fluid inside the gas turbine is higher, its pressure is significantly lower. The pressure of the working fluid of the gas turbine is usually only a few MPa, but the steam pressure of the current ultra-supercritical steam turbine can reach 30 MPa, resulting in the available specific enthalpy drop of the gas turbine being about 1/5 to 1/3 of that of the steam turbine. In general, gas turbines are usually inferior to steam turbines in terms of output power, but considering the high maneuverability and automation management of gas turbines, they still have certain application prospects in the field of large surface ships. Based on the above, the relevant technical parameters of various marine main engines are shown in Table 1.

6 Technical characteristics and development overview of marine combined power units

6.1 Origin of marine combined power units

The above marine power units differ in power, speed, maneuverability, economy, weight and size. For civil ships, the main consideration is economy, and the shortcomings can only be improved by appropriate measures. For military ships, combat effectiveness is the main goal, and more attention is paid to improving the power of the unit to improve the speed and maneuverability of the entire ship.

According to relevant statistics (Table 2), during navigation, surface ships are in cruising (low speed) conditions most of the time. At this time, the power output of the power unit usually does not exceed 25% of the total power, so a unit with lower power, longer service life and lower fuel consumption rate can be selected for operation. In the event of a war or actual combat exercise (the sailing time of surface ships under such conditions only accounts for about 3% of the total sailing time), another acceleration unit with higher power and correspondingly higher fuel consumption rate can be used. At the same time, the cruise unit and the acceleration unit can also be put into operation together to output higher power and meet the high speed requirements. Such a device is usually called a combined power unit, which can be used to balance the economic requirements of surface ships under cruising conditions and the high maneuverability requirements during combat.

6.2 Overview of the development of combined power units and their combination types

6.2.1 Overview of the development of combined power units

So far, combined power units for surface ships have been developed for a long time. Relevant experience shows that whenever a new type of thermal engine comes out, a new type of combined power unit composed of this type of unit and other existing main engines will often appear. In actual use, as the technical performance of new thermal engines improves, a single type of power unit will gradually gain an advantage and replace the previously used combined power unit. Until more advanced thermal engines appear, they will be combined with existing main engines again to produce a more novel combined power unit. This phenomenon repeats itself endlessly.

Specifically, the steam engine is a type of thermal engine that was born during the Industrial Revolution and has played an important role in human history. By the end of the 19th century, the steam turbine came into being, which is a new type of steam power unit. The earliest combined power unit in history consisted of a steam engine and a steam turbine. The design concept of this type of combined power unit is as follows: Due to the limitation of the clearance volume and cylinder stroke of the steam engine, the high-temperature steam cannot be fully expanded in the cylinder, so a steam turbine is arranged behind the steam engine. The steam that has done work in the steam engine enters the steam turbine to expand again, thereby recovering part of the steam energy. In this way, the cascade utilization of energy is achieved, and the power and thermal efficiency of the unit are effectively improved.

However, with the development of technology, steam engines have gradually withdrawn from the stage of history, and various large ships are more inclined to use a single type of main engine such as steam turbines. In order to carry forward the technical advantages of steam turbines and overcome their disadvantages, in addition to continuously improving the technical performance of the steam turbine body, it can also form a combined power unit with other types of thermal engines. After World War II, with the gradual improvement of gas turbine technology, its excellent power performance has also attracted widespread attention, and a series of combined power units with it as an acceleration unit have gradually emerged.

At present, there are mainly the following types of combined power units dominated by gas turbines.

1) Steam-combustion combined power unit. This type of combined power plant uses a small steam turbine as a cruise device and a gas turbine as an acceleration device. Compared with a single steam turbine power plant, this type of combined power plant has been significantly improved in terms of size, weight, and starting acceleration performance.

2) Fuel-fuel combined power plant. Fuel-fuel combined power plants are divided into two types: fuel-fuel combined power plants and fuel-fuel alternating power plants. The cruise unit and the acceleration unit of this device are both gas turbines. The cruise gas turbine can economically provide the low power required for cruising, and operate as an acceleration unit under high-speed conditions. This system has the advantages of flexible operation, high power, and light weight, but the device is expensive, and the intake and exhaust ducts occupy a large space on the deck, affecting the layout of the entire ship.

3) Diesel-fuel combined power plant. This type of combined power plant is divided into two types: diesel-fuel combined power plants and diesel-fuel alternating power plants. This device uses a diesel engine as a cruise unit and a gas turbine as an acceleration unit. Diesel engines are used in cruising and reversing, and gas turbines are used in high-speed navigation. This type of combined power unit has the advantages of low fuel consumption, good acceleration and good reliability.

6.2.2 Main combination types of combined power units

Taking into account the outstanding technical advantages of gas turbines, the combined power units composed of them are shown below and summarized in Table 3.

6.3 Main technical features of combined power units

In general, the combined power unit has the following technical features.

1) Since a lighter and more maneuverable gas turbine is used as an acceleration unit, and thus provides most (or even all) of the power under high-load conditions, the total weight of the power unit of the entire ship can be reduced accordingly.

2) Since a more efficient and economical cruising unit is used, the endurance of surface ships can be greatly improved.

3) Since two types of independent units are used, the reliability of the power unit is improved.

4) To realize the reverse process of surface ships, the combined power unit is more suitable for matching with related systems such as pitch propellers, reducers, and electric propulsion. At this time, any main engine can independently drive the propeller. However, it should be noted that if a main engine that can achieve reverse rotation (such as a low-speed diesel engine and the reverse turbine mentioned in 3.3 of this article) is used, it will often lead to mismatch of transmission power or other more complex technical problems, thereby reducing the reliability of the system.

6.4 Overall development trend of combined power units

As mentioned above, due to the different performance characteristics and applicability of several types of thermal engines, it is not possible to make the related main engines into an ideal combined power unit by simply combining them in pairs.

As for the current combined power units, gas turbines are mostly used as acceleration units. Taking the COGOG type combined power unit as an example, the cruise unit usually uses a heavy gas turbine with a larger unit weight, smaller power, lower fuel consumption rate and longer life, while the acceleration unit usually uses a light gas turbine with larger power, higher fuel consumption rate and shorter life.

Another example is the COSAG type combined power unit mentioned above. With the increase in diesel engine power and the improvement in gas turbine efficiency in recent years, as well as the actual use effect and reliability of the reverse steam turbine in the combined state of the unit, it is urgent to improve. Therefore, the COSAG type combined power unit is gradually replaced by the CODOG type combined power unit and the CODAG type combined power unit.

7 Technical characteristics and specific types of marine nuclear power units

7.1 Overview of marine nuclear power units

The main part of a nuclear power unit is an atomic reactor, which is equivalent to the furnace and combustion chamber of a boiler. Nuclear power units generally use 235U as the nuclear fuel of the reactor. Nuclear-powered warships usually use high-concentration nuclear fuel with a concentration of more than 20% to 40% to reduce the size and weight of the device. Nuclear-powered merchant ships, from the perspective of economy, mostly use low-concentration nuclear fuel with a concentration of less than 5%. Nuclear power plants can significantly improve the endurance of the ship’s main engine, and there is no need to inhale air and discharge exhaust gas, which is of particular significance to submarines. Therefore, nuclear power plants are first widely used on submarines. Nuclear power plants also have good application prospects on large ships such as aircraft carriers and cruisers. Due to the harm of radioactive substances in nuclear-powered ships to the human body and the pollution of port waters, a huge lead protective layer and a complete set of safety protection measures must be used, which is expensive and the testing and management technology is complex. Although it is also used in civilian ships, it has not been widely promoted and is still in the development stage.

7.2 Main technical features of marine nuclear power plants

The large-scale application of nuclear reactors has opened up broad prospects for the development of marine power plants, and its technical advantages are mainly as follows.

1) A tiny amount of nuclear fuel can be consumed to obtain huge energy. Ships using nuclear power plants can travel extremely long distances at a high speed. For example, a nuclear power plant with a power of about 11,040 kW (15,000 PS) consumes only 15 to 18 g of nuclear fuel in one day and night. The first nuclear submarine of the United States, the Nautilus, can sail around the world underwater without refueling. The Soviet nuclear-powered icebreaker Lenin can sail continuously for a year without refueling. After using nuclear power plants, the endurance of surface ships is greatly improved, and the space saved can be used to carry more weapons and equipment, improving the combat capability of the entire ship.

2) No air consumption. The nuclear reaction process does not require the participation of air. This feature is unmatched by any other type of power plant, especially for submarines. By using nuclear power plants, the combat effectiveness of submarines can be significantly improved, and they can be hidden in the deep sea for a long time, making them difficult to be discovered by the enemy. The characteristic of nuclear power plants that they do not consume air also has certain advantages for surface ships, because there is no need to set up air intake and exhaust ducts, and no high-temperature smoke will be generated, which correspondingly improves concealment. In a nuclear war, the risk of inhaling radioactive smoke from the air intake is also reduced, making it easy to carry out nuclear protection.

But at the same time, nuclear power plants also have certain disadvantages, mainly as follows.

1) Large weight and size. Since the nuclear reaction process will release a large amount of radioactive substances, which will cause serious harm to the human body and cause certain pollution to the open sea, offshore waters and docks, it is necessary to set up barriers weighing hundreds of tons or even thousands of tons to prevent the escape of radioactive substances, which makes the size and weight of the entire power plant larger.

2) Nuclear power plants are expensive and the operation and management technology is complex, which to a certain extent restricts their large-scale promotion.

For the above reasons, nuclear power plants are mainly used for large surface ships and submarines, and their development in the field of civilian ships is relatively slow.

7.3 Specific types of marine nuclear reactors
To ensure the safety of onboard personnel, nuclear-powered ships usually have stricter protection requirements for radioactivity than land-based nuclear power plants. Surface ships may encounter collisions, groundings, fires and explosions during navigation, or may be sunk by accidental attacks from weapons such as torpedoes and missiles. Under the premise that related accidents are likely to occur, in order to reduce the spread of nuclear pollution, ship nuclear power plants should have the function of permanent shutdown and need to be equipped with solid reactor barriers. According to the special requirements of ships for nuclear power plants, the pressurized water reactors currently used are mainly of the following three types.

7.3.1 Highly enriched uranium plate fuel element reactor

This type of reactor uses plate fuel elements with an enrichment of 235U of more than 20%. The plate element has a large heat dissipation area, a compact core layout, a small volume, and a high output power per unit volume. This can reduce the size of the reactor shell and make the equipment layout more compact. However, its technical disadvantages mainly lie in the high concentration of nuclear fuel required, and the high cost of design, construction and operation.

7.3.2 Low-enriched uranium dispersed pressurized water reactor

The structure of this type of pressurized water reactor is roughly the same as that of a land-based nuclear power plant pressurized water reactor. It consists of a steam generator, a reactor, a primary coolant pump and a pressurizer. The relevant components are connected by pipes to form a high-temperature, high-pressure closed loop. The secondary loop system and equipment are similar to conventional marine steam power plants.

7.3.3 Integrated pressurized water reactor

Take the German “Otto Hahn” marine pressurized water reactor as an example. It adopts an integrated structure, a simple primary loop system, compact equipment, and a small reactor safety shell size, which is suitable for surface ships. In addition, the core is filled with cooling water and has good natural circulation performance. When the cooling pump stops in the primary loop, the natural circulation of the coolant can still be relied on to maintain the cooling process of the core. The disadvantage of the integrated pressurized water reactor is that the reactor, evaporator and main pump are connected together, which makes the internal structure of the reactor complex and increases the difficulty of design, manufacturing and maintenance.

8 Marine electric propulsion device

Electric propulsion devices can also be used for ship propulsion. This type of device obtains electrical energy through various means, and then the motor drives the propeller to provide propulsion power for the ship. Its characteristics are that the propeller speed can be adjusted arbitrarily to meet the navigation needs under various working conditions, and it is simple to operate and easy to manage. It is more suitable for some ships with special requirements, such as submarines, scientific research ships, ferries, etc. The biggest advantage of the electric propulsion device is good maneuverability. The minimum speed of its motor can reach less than 1/10 of the rated speed, and the ship can sail at extremely low speeds. In addition, the startup and forward and reverse reversing time of this type of device is also short. The main generator and the motor-driven propeller can each operate under the best working conditions, and it is easy to remotely control and manage, and the vibration and noise of the entire device are relatively small.

Generally speaking, the unit used for power generation consists of a prime mover and a generator. The prime movers mainly include steam turbines, diesel engines and gas turbines, which are basically the same as the main engine types of ships. As mentioned above, due to the relatively small power of diesel engines, they are generally used for the main and auxiliary generator sets of military auxiliary ships, or the auxiliary generator sets of military ships. Steam turbines have the advantages of high speed and high power, and the technology is relatively mature, but they also have disadvantages such as large size and weight, low efficiency, large ship area, and difficult layout. If they are used in generator sets, the power density of the entire generator set will be reduced. In addition, due to the high speed of steam turbines, if a 50 Hz AC power grid is used, a huge gearbox is required to reduce the speed, which will increase the noise of the entire ship accordingly. Gas turbines have good prospects in this field. At present, many ships’ electric propulsion systems choose gas turbines as the prime movers for power generation.

9 Overview of the development history of marine power plant technology

Since the steam engine was used in marine power plants, it quickly occupied a certain dominant position. Until the First World War, steam engines still dominated civilian and military ships around the world and were the most important source of power for ships. However, the heyday of steam engines was coming to an end. Due to the large size and low thermal efficiency of steam engines, they were gradually replaced by diesel engines and steam turbines during the First World War and the Second World War. At the same time, because the fuel consumption rate of low-speed diesel engines was significantly lower than that of steam turbines, they could burn low-quality fuel and had high reliability, so they gradually replaced steam turbines in the field of civilian ships.

For ships with larger displacement, steam turbines were almost all used, while small ships were more inclined to use diesel engines. It was not until 1947 that a new rival for diesel engines appeared. After the first British ship propelled by gas turbines was tested, it began to compete with diesel engines and steam turbines that dominated at the time.

Although it was highly economical and had achieved remarkable results in the field of marine power plants, the diesel engine itself was limited in power and could no longer meet the power performance requirements of large ships. In contrast, although the steam turbine has a simple structure and high power, it needs to be equipped with a large steam generating device (such as a boiler, etc.) and auxiliary systems, and its economy and maneuverability are poor. This contradictory situation continued until the gas turbine came out. When the gas turbine was widely popularized in the field of military ship power, the above problems were gradually solved. Gas turbines can not only build a single model of power unit, but also be matched with other power units such as diesel engines to form a new combined power unit. Ships with complex cycle gas turbines as power units not only have good economic indicators, but also have high overall power, which can be said to be the best of both worlds.

Precisely because gas turbines have the advantages of easy starting and good maneuverability, they have received widespread attention and are likely to be “latecomers”. However, since gas turbines work under high temperature and high pressure conditions, they have high requirements for fuel quality and their thermal efficiency is significantly lower than that of diesel engines, so they are rarely used in civilian ships.

Nuclear power plants use nuclear reactors to obtain high-temperature, high-pressure steam. Nuclear reactors generate high energy through controllable nuclear fission chain reactions, which are absorbed by the continuously circulating cooling water. Then the heat is transferred to the water in the second loop through the steam generator, which turns it into steam and then goes to the steam turbine to do work. At present, nuclear power plants are mainly used in large surface ships and submarines.

Countries around the world have also conducted relevant research on the application of nuclear power plants in the field of civilian ships. The United States has successfully tested it on the “Savannah” ship; the Soviet Union has also used nuclear power plants on the icebreaker “Lenin”. Since then, Germany and Japan have also built nuclear-powered civilian ships. After a period of trial voyage, the relevant ships were forced to stop sailing for legal and public opinion reasons. Due to concerns that radioactive substances will pollute waterways, ports and urban environments, many ports refuse to allow nuclear-powered ships to enter the port. There is also a lack of proper treatment methods for nuclear waste after the use of nuclear fuel, which to a certain extent limits the application of nuclear power plants in civilian ships.

With the rapid development of science and technology, new marine power devices continue to emerge, such as fuel cells, magnetohydrodynamic propulsion, solar cells, etc. Among them, the novel magnetohydrodynamic propulsion has a good prospect. This propulsion method is a superconducting electromagnetic propulsion. Its principle is to install superconducting magnets on the ship, and then pass current through the seawater, using the magnetic field generated by the superconducting magnets and the force generated by the current in the seawater to propel the ship forward. The characteristics of this propulsion form are simplified transmission mode, no rotating parts (no propeller and shaft system), large thrust, and high speed.

10 Main technical requirements for marine power devices

In order to ensure the navigation capability of marine power devices, when selecting the main engine, the following technical performance needs to be fully met, which can also serve as an important reference for the selection of the main engine.

10.1 Reliability

After being put into operation for a period of time, electromechanical equipment will inevitably fail. For marine power devices, reliability is extremely important. Reliability has two meanings: one is the vitality of the power unit, which mainly refers to the ability of the power unit to maintain operation after being affected by external factors. The worse the conditions it can withstand, the stronger its vitality; the other is the time the power unit is in normal operation. The longer the normal operation time, the higher the reliability, and vice versa.

For surface ship power units, multi-engine multi-propeller or multi-engine parallel propulsion methods are often used, and their reliability and vitality are significantly higher than the single-engine single-propeller propulsion method. Taking the dual-engine dual-propeller propulsion method as an example, when one of the main engines or the propeller matched with it fails seriously, the other main engine and propeller can still operate normally, and the ship’s propulsion force is not completely lost. Therefore, from the overall propulsion effect, the dual-engine dual-propeller propulsion method has stronger vitality. Therefore, surface ships mostly use dual-engine dual-propeller or multi-engine dual-propeller transmission methods.

10.2 Maneuverability

Maneuverability refers to the ability of a marine power unit to transition from one operating condition to another, such as starting, accelerating, braking, reversing and merging. The performance of the power unit’s operating condition conversion directly affects the ship’s ability to leave the dock, sail in ice areas, sail in foggy weather, and avoid emergencies.

10.2.1 Startability

The quality of startability is closely related to the type of main engine. As for the start-up time of a diesel engine, it mainly depends on the system with the longest operation time among the auxiliary systems such as fuel, lubricating oil, cooling water, and starting air. In order to improve the maneuverability of a diesel engine, measures such as cylinder warming are usually adopted.

Compared with steam turbines, diesel engines require a certain flow of high-temperature steam during the start-up process of steam turbines, so it mainly depends on the process of steam parameters reaching the specified state after the boiler is ignited. Although some boilers can supply steam quickly, the average start-up time of steam turbines is still longer than that of diesel engines.

Compared with diesel engines, gas turbines have a relatively shorter start-up time, but overall, there is no significant difference between the two.

10.2.2 Acceleration

Acceleration is also related to the type of main engine. The first condition for shortening the acceleration time is that the main engine needs to increase the power to the maximum value in a short time. The second is whether the propeller can fully absorb the power output of the main engine during the acceleration process and convert it into external propulsion power.

For the main engine, the main factors affecting the acceleration time are the weight and thermal inertia of the components. Lighter weight and lower thermal inertia are conducive to acceleration. Generally speaking, the heat-bearing components of gas turbines are relatively light and small, so they have better acceleration performance.

For propellers, since the speed of fixed-pitch propellers is subject to certain restrictions, it will increase accordingly with the increase of ship speed. Therefore, as the speed of fixed-pitch propellers increases, the power of the main engine also needs to be gradually increased to achieve a reasonable match. The adjustable pitch propeller is not limited by its own speed when absorbing power, and can generate greater thrust in a short time, so its acceleration is better than that of fixed-pitch propellers.

10.2.3 Braking and reverse performance

The performance of a ship in braking and reverse mainly depends on the propeller and the corresponding transmission mode. For the combination of a reversible low-speed diesel engine + fixed-pitch propeller, the low-speed diesel engine must first stop injecting fuel and reduce the speed to a certain range before starting to reverse, so the reverse time is generally longer.

For combinations such as non-reversible main engine + clutch + fixed-pitch propeller, it mainly depends on the performance of the clutch. If the system uses a friction clutch, the reverse time of the ship mainly depends on the temperature increase of the friction parts when the clutch reverses. The higher the mechanical load and thermal load that the clutch can withstand when reversing, the more conducive it is to improve emergency braking and reverse capabilities.

For combinations such as non-reversible main engine + adjustable pitch propeller, because the ship does not need to stop the main engine when braking, and reverse thrust can be generated by adjusting the pitch of the blades, its braking and reverse performance is relatively good.

11 Analysis of the application prospects of marine power units

In recent years, diesel engines have become the main power source for various types of ships. In today’s era of increasingly scarce energy, considering the high thermal efficiency of diesel engines and the promotion of low-quality oil technology, diesel engines will continue to occupy an important position in ship main engines in the next few years.

Middle and small surface ships are mostly powered by diesel engines. These ships include submarine hunters, minesweepers, patrol boats, gunboats, torpedo boats, missile boats and auxiliary ships. Some large ships such as frigates and destroyers also use diesel engines or diesel-fuel combined power units as power sources. In diesel-fuel combined power units, diesel engines are mostly used as cruise main engines to take advantage of the low fuel consumption rate to extend the ship’s cruising range. For submarines, except for nuclear power units, all conventional submarines use diesel engines as main engines, and some also use air-independent propulsion systems (AIP).

At present, due to the development of diesel engines and gas turbines, steam power units represented by steam turbines have given up their former comprehensive dominance and formed a three-legged tripod together. However, steam turbines still dominate large ships, especially aircraft carriers and nuclear submarines.

Due to the large fuel consumption of steam turbines, the number of newly built civilian steam turbine ships is decreasing year by year. In contrast, almost all liquefied natural gas ships use steam turbines to recycle and utilize the escaped natural gas as boiler fuel. In my country, steam turbines still dominate large and medium-sized ships, such as aircraft carriers and nuclear submarines. From the perspective of actual ship use, the outstanding advantages of steam turbines are safety and reliability, good maneuverability, ease of use, and good maintainability. Its disadvantage is poor economy. Under the same amount of fuel, the cruising range is short. This is a fatal weakness that affects the development of marine steam turbines. In order to enable the continued development of steam turbines, efforts must be made to improve economy.

As mentioned above, during and after World War II, diesel engines and steam turbines were widely used on ships. However, with the subsequent development of technology, there has been a trend of using gas turbines as the main engine. At present, gas turbines can be mainly divided into light gas turbines and heavy gas turbines according to their different structural types. Among them, light gas turbines are a type of new unit that is based on aviation gas turbines and further modified to adapt to ship navigation conditions. Heavy gas turbines are developed based on industrial gas turbines and are currently mostly suitable for large civilian ships, such as container ships, roll-on/roll-off ships and ferries, but due to the poor economy of this type of unit, its application is still in the experimental stage.

Marine gas turbines mainly follow the path of aviation modification. Under the conditions of the marine environment and the requirements of ship use, a large number of research and experimental work has been carried out from the aspects of improving economy and reliability, and many new technologies and processes have been adopted. The products have been updated many times and have reached a relatively mature stage, but compared with diesel engines, there is still a certain gap in economy. At present, both steam turbines and gas turbines are working hard to improve economy and reduce fuel consumption, and have achieved gratifying results.

In order to balance the economy under cruising conditions and the acceleration required for strategic maneuvers, surface ships generally use combined power units, including cruise units and acceleration units. Both units are connected to the main reducer by a clutch, and reverse and forward gearboxes or adjustable pitch propellers are used to implement reverse. The advantage of the combined power unit is that it has sufficient power, and the weight and volume of the unit are relatively small. It solves the contradiction between full-speed high power and cruising economy, and improves the endurance of the ship.

Nuclear power units can significantly improve the power and endurance of ships, and do not require air combustion, which is more suitable for underwater ships such as submarines. However, since radioactive substances can cause serious harm to the human body and pollute the surrounding environment, strict radioactive protection equipment must be set up for this. Although nuclear fuel occupies a small space in nuclear-powered ships, considering the required shielding layer and huge auxiliary equipment, the advantages are offset, and it is still possible to further increase the total ship load. In addition, nuclear power plants are rarely used in civilian ships due to their heavy weight, high cost, and complex construction, operation, and management technologies. At present, the application of nuclear power plants in civilian ships is more determined by factors such as the acceptance of the local port environment, international agreements, insurance agreements, and initial costs, rather than simple technical feasibility.

In summary, the application prospects of marine power plants are shown in Table 4.

According to the above situation, civilian ships will still use diesel engines as the main power plant for a period of time. For large civilian ships, low-speed diesel engines are generally still the main power plant, while for ships with limited cabin height such as ro-ro ships, passenger ships, ferries, etc., medium and high-speed diesel engines are more suitable. Among them, inland ships are still mainly medium- and high-speed diesel engines, while large ocean-going ships are mainly low-speed diesel engines. In recent years, the fuel consumption rate of medium-speed engines has been close to that of low-speed diesel engines. They have the advantages of high waste heat utilization efficiency, small size, light weight, low cost, and good prospects.

According to my country’s situation, medium and small surface ships, submarines and auxiliary ships will still be mainly equipped with medium and high-speed diesel engines. Large submarines will focus on the development of nuclear power units and still use steam turbines as the main engine. At the same time, gas turbines have significant performance advantages and have gradually become the standard power unit for domestic and foreign military ships. Various combined power units with gas turbines as acceleration units should also be paid attention to and developed. At present, it is urgent to strengthen the research and development of high-power gas turbines for ships.

12 Prospects for marine power units

To sum up the above, diesel engines have the advantages of high thermal efficiency, good economy, large power range, compact structure, few auxiliary equipment, and can directly drive propellers. However, with the gradual increase in ship tonnage, marine diesel engines are required to develop in the direction of high power. To increase the power of diesel engines, the only way is to increase the cylinder diameter, increase the number of cylinders or use multiple models. This will inevitably increase the weight and volume, making manufacturing difficult and expensive. Therefore, diesel engines are usually not used as the main propulsion power unit on large ships. After years of development, marine diesel engines have reached a relatively high technical level. However, for medium and small surface ships, conventional submarines and civilian ships, diesel engines are still the main power source. Low carbon emissions are a serious challenge facing diesel engines at this stage. With the restrictions on emissions from marine diesel engines, it is more difficult to improve their economy, which is also a new topic in the future development of marine diesel engines. In general, due to the significant characteristics of diesel engines, their technological progress has greatly promoted the development and revolution of ship technology and opened a new chapter in the development of marine power units.

Steam turbines have high power, small size, light weight, stable operation, long life, high reliability, low lubricating oil consumption rate, strong overload capacity, can burn low-quality fuel, low vibration and noise, but low thermal efficiency, complex management, difficult processing and manufacturing, high cost, must be equipped with a reduction gearbox and a large main boiler, and must drive the propeller through a reduction device. Currently, they are mostly used in large oil tankers, aircraft carriers, and nuclear submarines.

Compared with the above power units, gas turbines are small in size, light in weight, occupy the least cabin space and displacement, and have the advantages of low vibration and friction, easy management and maintenance, rapid startup, and can reach maximum power within a few minutes after startup. Therefore, they are more suitable for military ships. However, due to the disadvantages of low thermal efficiency, poor economy, short life, high requirements for metal materials and inability to reverse, they have not been widely promoted on civilian ships, and are mostly used in high-performance ships such as hovercraft.

In the field of surface ships, there are combined power units that use gas turbines in conjunction with diesel engines, steam turbines, and nuclear power units. This type of power unit appeared after World War II and can give full play to the advantages of various main engines and make full use of the advantages of gas turbines.

Nuclear power units have advantages that other power units cannot match, but they need to be equipped with careful and heavy protective measures, and are expensive, and the construction and testing management technology is complex. Therefore, they have not been widely used in civilian ships for the time being, and are mostly used in large surface ships such as aircraft carriers and submarines. With the increasing depletion of energy such as oil, nuclear power units are expected to be further promoted in the field of civilian ships.

13 Conclusion

In summary, marine power units are developing in the direction of diversification and high power. Medium and small civilian ships mostly use medium and high-speed diesel engines. Large and medium-sized civilian ships mostly use medium and low-speed diesel engines. Steam turbines have the advantages of mature and reliable technology, good maintainability, and long service life, but their disadvantages are poor economic efficiency and complex installation. Although gas turbines have the disadvantage of short service life, ships can make full use of the advantages of gas turbines that they can start quickly and obtain high power in a short time, so they still have good prospects.

Nuclear power plants will become an important source of marine power. Once nuclear fuel is installed, the ship can sail for several years. It is very suitable as a power source for large ships, and steam turbines will also be used as the main engine. Electric propulsion will become an important propulsion type for future ships. It can provide electricity through batteries, generators driven by prime movers, and electrochemical power generation equipment such as fuel cells.