What is "air-independent propulsion" submarine technology?

Air-independent propulsion refers to the technology that can drive a submarine for a long time without obtaining oxygen from the outside air. The self-sustaining power of submarines using this technology is more than twice that of ordinary diesel-electric submarines, that is, the continuous dive time and dive distance are longer, but it is still much shorter than that of nuclear submarines. Its cost is between ordinary diesel-electric submarines and nuclear submarines. Closed cycle diesel engine (CCD)AIP, Stirling engine (SE)AIP and fuel cell (FC)AIP. These three AIP schemes are adopted by various naval powers respectively; For example, Britain and the Netherlands adopted CCDAIP scheme, Sweden adopted SEAIP scheme, and Germany adopted FCAIP scheme. At present, only Russia, Germany, Sweden, France, Spain and China clearly possess this technology. 1935, S.A.Bazilevskiy, the designer of the Soviet Central Design BureauNo. 18, proposed to use an air-independent system in the submarine, and carried out experiments on the M-92 submarine, and obtained a lot of experimental data. But at that time, the technology was not mature, and the designer would not rely on air technology, but only put it forward as an experimental topic. In World War II, the German company Walter tried to use compressed hydrogen peroxide as the underwater oxygen source of diesel engines in submarines. Its principle is that when working underwater, hydrogen peroxide with potassium permanganate as catalyst is heated by the thermal energy of gas turbine, thus producing oxygen for diesel engine. Later, Germany built several submarines that did not rely on air propulsion. One of them, U- 1407, was abandoned at the end of the war and renamed as HMS meteorite by the British after the war. After that, Britain built two identical submarines based on meteorites, HMS Explorer and HMS Excalibur. After the war, the Soviet Union resumed the experiment of hydrogen peroxide fuel, but it finally gave up because of the extremely unstable transformation of hydrogen peroxide into oxygen. Both the Soviet Union and Britain started their own nuclear power research, and gave up further research after seeing that the United States already had nuclear reactors available for submarines. However, this propulsion system is still used in Soviet and British torpedoes. However, this unstable fuel still caused the tragedies of the Royal Navy Sidon and the Russian Navy Kursk nuclear submarines. Japan itself did not claim to own this technology, but imported Stirling engines from Sweden, which were produced by Kawasaki Heavy Industries. Japan itself is good at fuel cell technology, but the current proton exchange membrane fuel cell technology can not meet the needs of large Japanese submarines, so it has not been adopted. It is expected that the next generation submarine of Black Dragon will use fuel cells with hydrogen storage metal technology to meet the needs. In addition, it is said that in response to Japan's related actions, South Korea is also applying to Sweden to introduce the same type of Stirling engine. The United States has not developed the AIP propulsion system because it can afford expensive nuclear submarines, and the demand for shallow water combat capability is not urgent. On the other hand, nuclear-powered submarines also belong to AIP, but nuclear submarines are generally classified independently and are not included in ordinary diesel-electric AIP submarines. Therefore, China, the United States, Russia, Britain and France, which have nuclear submarines, have no urgent need to develop AIP technology, and most of them are in the period of shortage of funds or export models. Because Germany and Japan were defeated in World War II, the Allies imposed a ban on the production, introduction and possession of any form of nuclear weapons (including nuclear-powered submarines) against the defeated countries, such as Japan's postwar national policy "postwar constitution" and "three non-nuclear principles". Compared with the nuclear-powered submarines of Britain, France, the United States and Russia, Germany and Japan are not satisfied that diesel-electric submarines are only used for offshore defense operations. Therefore, based on the traditional diesel-electric submarine power technology, Germany and Japan have further developed air-independent propulsion engines and become the main underwater combat forces of the German and Japanese navies.