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Submarine Stirling Engine

The Stirling engine, a new type of engine for submarines, is an external combustion engine with an internal cycle. The 039A/B AIP (Air Independent Propulsion) submarines of the Chinese Navy use the Stirling engine system. At present, the fourth-generation non-nuclear submarines of various countries in the world are basically equipped with AIP devices as auxiliary power devices, and some advanced submarine-building countries have begun to develop the fifth-generation non-nuclear submarines. The biggest feature of the fifth-generation non-nuclear submarines is that they use a single full-load engine both on the surface and underwater.

The AIP device has been used as the main power device of the submarine and is the only power device. In the international arms market, it is difficult to find buyers for non-nuclear submarines without AIP devices. The Submarine Non-Dependent Air Propulsion System (AIP) is a propulsion system used in submarines that allows submarines to operate for long periods of time without surfacing or using a snorkel to enter the air. Unlike traditional propulsion methods that involve diesel engines and batteries that require oxygen, propulsion systems that do not rely on air use alternative means that do not require oxygen to generate electricity. Air-independent propulsion provides longer underwater range, greater stealth and reduced likelihood of detection.

Everyone has heard of Watt, who invented the steam engine by observing a teapot boiling water. To put it simply, a steam engine is a machine that uses gas expansion to do work, and needs to be heated from the outside, so it is called an external combustion engine. In contrast, in cars, gasoline is burned in the cylinder, and the hot gas from the combustion pushes the piston, which is also called an internal combustion engine.

British physicist Robert Stirling patented the first practical example of a closed-cycle hot air engine in 1816, and it was suggested by Fleeming Jenkin as early as 1884 that all such engines should therefore generically be called Stirling engines. The concept of mechanically manipulating the ideal gas laws to convert heat into motion or vice-versa was first patented by Robert Stirling. Since that time several designs, most utilizing multiple pistons, have emerged including some designs utilizing pressure waves in lieu of a displacer with only a single piston.

The basic Stirling engine includes a trapped gas that is heated or cooled which then expands or contracts (according to the ideal gas laws) which pushes or pulls on a piston which then drives a crankshaft. The crankshaft is typically coupled to a flywheel and an output shaft. The output shaft delivers usable mechanical force relative to the initial temperature differential and amount of heat transferred.

Current commercial designs utilize a piston style displacer to move the working gas from a heating chamber to a cooling chamber and back. Common designs use multiple internal seals and two or more pistons. Current designs are complex and difficult to manufacture making them relatively high cost. The greater efficiency, reliability, lifespan, cleanliness, and flexibility that Stirling engines demonstrate compared to internal combustion engines has previously been sacrificed in favor of the faster start up, control response, greater power density, and ease of manufacture of competing engines. However, the inherent advantages of the Stirling engine allows it to compete successfully in various specialty niches of the engine market, such as satellite power production, waste heat recovery, cryogenics, solar power conversion, space craft, and submarines, where faster start up, control response, greater power density, and ease of manufacture are not the critical criteria in engine selection.

The Stirling engine is an external combustion engine. It also does work by expanding the gas. However, after the expansion, the exhaust gas is not discharged. Instead, it is sent to another cylinder for cooling and compression. The cooled gas is then sent back to be reheated and expanded. This cycle repeats, driving the piston to do external work. Generally speaking, the greater the temperature difference between the heating cylinder and the cooling cylinder, the higher the engine efficiency. Affected by materials and the environment, this type of engine will not be particularly efficient, just like an internal combustion engine. It is generally only about 30%, and the better ones can reach more than 40%.

Compared with diesel engines, Stirling engines have high working efficiency, low working noise, low fuel requirements, and simpler maintenance. It seems to be a perfect substitute for diesel engines. However, In 2018, the 11th Japanese Soryu-class submarine , the Huanglong, was launched. It abandoned the Stirling AIP engine and used lithium-ion batteries as the main power. Conventional submarines have low radiated noise and are flexible and agile, and are still the main force for offshore operations. However, conventional submarines often need to sail with snorkels, and the snorkels exposed above the water are easily discovered by anti-submarine detection equipment. Since the 1970s, with the rapid development of integrated anti-submarine detection technology and integrated weapon systems, the threat to conventional submarines has increased day by day.

Diesel-burning submarines use diesel engines to drive propellers, or use diesel engines to generate electricity and then use electricity to drive propellers. However, for submarines, there is a very big problem that needs to be solved with diesel engines, that is, diesel combustion requires air, and submarines are confined spaces with limited air. Either they need to surface regularly for ventilation, or they need to install snorkels to extend out of the water for ventilation. Because they rely on air, submarines cannot stay safe underwater, and their safety will be affected. The noise of the diesel engine is really difficult to eliminate. The noise of the Stirling engine is the smallest among these heat engines, which is very important for submarines.

A steam turbines heat liquid water, and the water turns into steam and is ejected to drive the turbine to rotate and do work. Today's steam turbines are much more powerful than Stirling engines. But there are also disadvantages in submarines, because steam cannot be discharged directly, it needs to be condensed into water and then circulated back for heating. This greatly increases the complexity of the system.

Stirling engines use high-pressure gas inside, not water. Gas will expand as long as it is heated, and water must first become steam before it can expand significantly. Obviously, the heating requirements of Stirling engines are much smaller than those of steam turbines that use water. The compression and expansion of the Stirling engine are similar to those of an ordinary internal combustion engine, but the power piston moves in a closed circuit helium or hydrogen working gas system, and the heat is continuously transferred to the circulation loop through the heat exchanger in the combustion chamber.

The hot cylinder has a heat absorbing plate to help absorb heat, and the cold cylinder has a heat sink to help dissipate heat. The two cylinders are connected through a regenerator. The cylinder is filled with gas as a working medium, and the working medium will not leak out. The hot piston can move back and forth in the hot cylinder, and the cold piston can move back and forth in the cold cylinder. There is a heating source outside the hot cylinder to heat the gas in the cylinder, and there is a cooling source outside the cold cylinder to absorb the heat of the gas in the cylinder.

As a closed-cycle reciprocating power machine, the theoretical efficiency of the Stirling engine is equal to that of the Carnot cycle, which is the highest efficiency that can be achieved by all thermodynamic cycles. At the same time, as a closed-cycle power machine with external heating, the Stirling engine can be combined with any heat source such as conventional energy and nuclear energy. Its structure is flexible and changeable. It has significant advantages such as compact structure, simple system, rapid start-up, and modular layout within the megawatt power level range.

The Stirling engine is known for its application on submarine propulsion. Swedish Gotland-class submarines feature Stirling engine air-independent propulsion (AIP) systems, enabling them to avoid going to the surface to charge the batteries, and thus increasing the submerged endurance from days to weeks. Sweden's A19 Gotland class submarines, with a submerged displacement of 1647 Tons, are equipped with two MTU diesel engines and two Kockums V4-275R Stirling Air Independent Propulsion units. The Stirling engines each provide up to 75 kW, giving the submarine the capacity for two weeks of air independent propulsion at a speed of 5 knots without snorting.

Although its power has been increased and the underwater sailing time has been increased, it has also brought a series of problems at the same time. For example, its working noise is relatively loud, and its heat power conduction is relatively slow, so it brings a certain delay to the working process of the entire power unit. But in general, due to its increased power and overall improvement in performance, the disadvantages are still less than the advantages.

The 711 Institute is the main research institution of China's conventional submarine power system. It has developed power systems for many types of submarines. In 1975, the older generation of researchers established the Stirling engine test laboratory at the 711 Institute. It is more efficient and has a wider range of energy applicability. Possessing this technology is of great significance to the country. At that time, only a few countries in the world were developing this type of engine, and the technology was strictly sealed from the outside world. There were no materials, no drawings, no physical objects, no laboratories, and no experience to draw on. After ten years, the older generation of researchers did not give up and finally developed a small solar Stirling prototype, taking a solid first step.

Another ten years later, the research team of the 711 Institute achieved a technological breakthrough with meager research funds and developed China's first 10-horsepower engine. It took developed countries decades and huge costs to reach the same technological level. In another ten years, the research team of the 711 Institute solved more than ten key technologies such as sealing, combustion, heat exchange, etc. through "continuous self-denial and independent innovation, broke through all technical bottlenecks, broke the foreign technology blockade, successfully developed an engineering prototype, and realized product trials".

On 26 April 2015, the People's Daily reported that in the field of special engines, the Special Engine Team of the Thermal Turbine Division of the 711 Institute of China Shipbuilding Industry Corporation had spent nearly 10 years to successfully develop a new engine with completely independent intellectual property rights. Compared with the most advanced foreign products of the same type, the power is increased by 117%, which is a world first and has now been included in the country's application plan for the next generation of ships.

The AIP system of the Type 039B submarine uses a Stirling engine, which was developed by China in the 1980s with the introduction of Swedish technology. According to reports, Sweden currently offers a variety of 4-275 engines to the international market, including 75 kW and 110 kW models, but the 75 kW model is currently sold to the international market. It is reported that China originally introduced the 75 kW version of the Stirling engine, which was successfully copied domestically and used in the Type 039B conventional-powered submarine.

Since the power of Stirling engines is relatively small, they are generally used to drive submarines to sail slowly underwater. However, China's original new generator technology can simultaneously meet the requirements of submarines sailing slowly underwater and charging the main batteries. Therefore, although the total power of the four Stirling engines installed is the same as that of Japan's "Soryu" and other AIP submarines that are also driven by Swedish Stirling engines, it has higher tactical flexibility.

At the same time, the new power generation system developed by the scientific research team of Academician Ma Weiming was adopted, which enabled China's AIP system to have the ability to charge the main battery while sailing underwater at low speed. Increasing the power of a single machine can reduce the number of thermal engine units and reduce weight; if the number of units is guaranteed, the charging efficiency will be doubled, making China's AIP submarines more flexible and closer to small. It is said that in 2013, China also developed a 9-meter-diameter high-strength pressure-resistant hull AIP with a compartment section for submarines, which can be used to upgrade the early series of 039 submarines.

High-power Stirling engines are particularly important. One of the newly developed Stirling engines in China has 320 kilowatts, which can allow a 2,000-ton submarine to achieve an economic speed of about 8 knots. Four engines have a power of more than 1,200 kilowatts, which can theoretically allow a 2,000-ton submarine to achieve a speed of more than 12 knots. With a high-power battery pack, it can also achieve a maximum speed of more than 20 knots. This gives it a certain competitive advantage over traditional submarines, so it is reasonable to say that this engine is a milestone engine

The Stirling engine is quiet and vibration free, so no vibrations spread out to the hull making the submarine silent in the water and therefore difficult to find. The CSSC did not mention the potential application of the Chinese Stirling engine on submarines. The basic prototype with a rated power of 320kW is about double that to the pair of Stirling engines on the Gotland, suggesting it might be sized for a 3,300 ton submarine, or roughly that of the Type 039 series Yuan-class attack submarines. A pair of such engines [as used on the Gotland] might power the Type 032 Qing should it enter series production.

In a highlight of China's efforts to domestically develop high-quality engines of different types, China made a major breakthrough in making the world's most powerful Stirling engine, a type of heat engine with applications in power generation and submarine propulsion. China has experienced shortcomings developing engines of different types but the breakthrough in the domestically developed Stirling engine is only one example of the country's rapid engine progress in recent years, a Chinese military expert who requested anonymity told the Global Times on 22 December 2021.

Developed by the No. 711 Research Institute under the China State Shipbuilding Corporation Limited (CSSC), the basic prototype of China's first large-bore Stirling engine successfully conducted the recent performance test. The prototype ran at a rated power of 320 kilowatts with a power conversion efficiency of 40 percent, making it the most powerful Stirling engine known around the globe, the CSSC said in a press release published in its WeChat account on Tuesday. The success of the prototype is a milestone and a major breakthrough China has made in key technologies and craftsmanship in developing such kind of engines and laid down the foundations for its development of megawatt-class Stirling engines in the future, the CSSC said.

The Stirling engine is a closed-cycle reciprocating power machine that takes heat supply from external sources, the press release explained, noting that the engine can be combined with any kind of heat source, either conventional or nuclear energy, in many configurations. A megawatt-class Stirling engine is tightly structured, has a simple system and can be started quickly and configured in modules. These will be significant advantages of the Stirling engine, the CSSC announced.

When used together with a sodium-cooled fast reactor, the Stirling engine can eliminate the risk of sodium-water reaction making it also a very safe choice. The engine can spawn a series of variations to suit the need of power supply, ranging from 100 kilowatts to several megawatts, and is very suitable to be developed into a portable micro reactor power generator used in special environments such as polar regions, remote islands and Gobi Desert regions, according to the press release.

The Stirling engine is also known for its application on submarine propulsion, observers said. Swedish Gotland-class submarines feature Stirling engine air-independent propulsion (AIP) systems, enabling them to avoid going to the surface to charge the batteries, and thus increasing the submerged endurance from days to weeks, according to an article published by Saab, the parent company of Kockums which built the submarines. The Stirling engine is quiet and vibration free, so no vibrations spread out to the hull making the submarine silent in the water and therefore difficult to find, according to the article. The CSSC press release did not mention the potential application of the Chinese Stirling engine on submarines.

Through the combination of single or multiple Stirling generator sets, it can meet the power supply needs of hundreds of kilowatts to several megawatts. It is particularly suitable for mobile micro-reactor power generation scenarios. In particular, when combined with metal reactors such as sodium-cooled fast reactors, the risk of sodium-water reaction in traditional sodium-cooled fast reactor power generation systems can be fundamentally eliminated, and it has inherently high safety.