How diesel submarines run underwater without engines needing air

Diesel-powered submarines face the fundamental challenge that their engines require oxygen to operate, yet they routinely travel underwater where air is unavailable. Naval engineers resolved this over a century ago by developing the diesel-electric propulsion system. In this setup, diesel engines run only when the submarine is surfaced or near the surface using a snorkel mast, powering generators that charge large onboard batteries. Once submerged, the diesel engines are shut down, and the submarine moves silently using electric motors powered by the stored battery energy. This separation of power generation and propulsion allows underwater operation while enhancing stealth, as electric motors produce far less noise than combustion engines. While diesel-electric submarines can operate underwater quietly, their endurance is limited by battery capacity. To recharge, they must surface or snorkel, which risks detection due to visible masts and exhaust. To address this, many modern diesel submarines incorporate air-independent propulsion (AIP) systems that generate electrical power without atmospheric oxygen, extending underwater endurance from about two days to several weeks at