In the mid-1980s Kockums installed Stirling engines and a LOX (Liquid Oxygen) system from AGA Cryo aboard the French 500 ton civilian research submarine Saga which has an operating depth of 600 meters and is fitted with a diving system for work at depths as deep as 450 meters.
In the late 1980s Kockums fitted a Stirling engine in the Royal Swedish Navy submarine Näcken. The submarine was put into drydock and cut in two. A fully outfitted eight meter section was the inserted.
Kockums V4-275R Stirling Air Independant Propulsion Unit (AIP) one of two fitted in each vessel.
The main feature of Kockum´s Air Independent Propulsion (AIP) System is the use of Stirling engines burning pure oxygen and diesel fuel in a pressurized combustion chamber. The combustion pressure is higher that the surrounding sea water pressure, thereby allowing the exhaust products, dissolved in seawater, to be discharged overboard without using a compressor. Oxygen is stored in liquid form (LOX) in cryogenic tanks.