How do the engines breathe in diesel submarines?

A diesel submarine is a very good example of a hybrid vehicle. Most diesel subs have two or more diesel engines. The diesels can run propellers or they can run generators that recharge a very large battery bank. They can also work in combination, one engine driving a propeller and the other driving a generator.

The sub must surface (or cruise just below the surface using a snorkel) to run the diesel engines. Once the batteries are fully charged, the sub can head underwater. The batteries power electric motors that drive the propellers. Battery operation is the only way a diesel sub can actually submerge.

The limits of battery technology severely constrain the amount of time a diesel sub can stay underwater. This is the huge benefit of using nuclear power in a sub. Nuclear generators need no oxygen, so a nuclear sub can stay underwater for weeks at a time.