On the water surface, the submarine is powered by two sets of MTU engines. While submerged, the Kockums-built Stirling engine AIP system is used to drive a generator for either propulsion or charging the batteries. A Stirling engine is particularly well suited for a submarine because the engine is nearly silent and can use the surrounding sea water as a heat sink to increase efficiency. Submerged endurance is dependent on the amount of liquid oxygen stored on-board and is described as "weeks". The class is characterized by its low acoustic signatures, extreme shock resistance, and a competent combat system.