Submarines are essentially of two types: conventional and nuclear. Conventional submarines use a diesel-electric engine, and must surface daily for oxygen for fuel combustion.  

• If fitted with an Air Independent Propulsion (AIP) system, the submarine needs to take in oxygen only once a week. 
• The state-of-the-art “air independent propulsion” (AIP) system that the Defence R&D Organisation (DRDO) is indigenously developing will be ready to drive the Indian Navy’s submarines from 2024 onwards.  

• Consequently, this AIP will not power the six new submarines that the navy is tendering in a Rs 45,000crore programme called Project 75-I.  
• Instead, those six boats (as the navy traditionally refers to submarines) will have AIP systems that the foreign vendor must offer. 

Source: Indian Express