Saab's A26 submarine is the most modern submarine programme on the planet. But earlier generations of Swedish submarines were already unique thanks to, among other things, a 200-year old discovery. The Stirling engine has undergone continuous development since its conception, and it is a major reason our submarines today are extremely quiet and vibration-free.
The Stirling engine was invented in 1816 and was named for Robert Stirling, who invented the first closed cycle air engine. The engine is also known as a hot air engine as it is powered by the pressure difference in the working fluid at different temperatures. Heat can be attained from a wide range of sources, and the Stirling engine is used in, among other applications, civil solar energy projects. In Swedish submarines, heat is derived from low-sulphur diesel which is combusted using liquid oxygen stored in tanks on board. The Stirling engines are integrated in complete modules which also contain oxygen and fuel systems, along with a generator.
The air-independent propulsion system enables Swedish submarines to have an inordinately long operating time underwater, without having to surface and reveal their location. Submarines equipped solely with diesel-electric propulsion systems are able to remain submerged for a few days before they are required to surface (or break the surface using a snorkel). The Stirling engine increases underwater duration to several weeks.
The technology is cost-effective in comparison to other air-independent technologies, such as fuel cells and nuclear power. And these are both highly complex and difficult to manage.
All Swedish submarines are equipped with Stirling engines. The first engine was installed on the Näcken submarine in 1988 and, in the years that followed, the system has evolved and much operational experience has been gained. The system is now in its third generation, Stirling Mk3, which boasts increased efficiency and power density, as well as a new control system and HMI. Two Gotland class submarines will be fitted with the new Stirling Mk3 in conjunction with mid-term modifications.