Within a diesel engine’s cylinder, compressed air raises its temperature to ignite the injected fuel, and the resulting force drives a piston that powers the vehicle’s main gear. The main component of this combustion mechanism is oxygen, which is abundant in the air around us. The process is ideal for trucks, machines and other land-based vehicles that can utilize natural oxygen in the air. However, how does this process work underwater? More specifically, how do diesel submarines get enough oxygen to burn fuel underwater?
Simply put, when a submarine is underwater, its diesel engines don’t run at all. Traditional submarines are hybrid vehicles that use a combination of diesel and electric power. On the surface, they use a diesel engine to power propellers and a generator to charge batteries. But once the submarine sank, those diesel engines were no longer used. Instead, their batteries power electric motors, which are more efficient and quieter. Their noiseless operation allows submarines to remain invisible underwater. The downside is that diesel submarines can only stay underwater for a short period of time – they must surface again to run their diesel engines and recharge their batteries. Nuclear-powered submarines are more valuable than diesel-electric submarines precisely because their nuclear generators do not require oxygen, allowing the submarines to remain underwater for long periods of time.
Read more: What actually happens when a submarine implodes?
How often does a diesel-electric submarine surface?
A close-up rendering of a submarine traveling deep beneath the surface of the water. – Sanserstudio/Getty Images
A typical diesel-electric submarine must surface (or “snorkel”) every two to three days to operate the diesel engine and replenish its power reserves. On the surface, submarines are more vulnerable because they are easier to detect and do not move as fast. Sweden’s fifth-generation submarines have an advantage in this regard. Although they are diesel-electric powered, these submarines use a unique “air-independent” propulsion system that can stay underwater for weeks at a time.
But even these advanced diesel submarines are no match for nuclear-powered submarines. They can stay underwater for months at a time—often up to 120 days—as they need to replenish their food stores. It’s no wonder the U.S. Navy is currently adding more next-generation nuclear-powered submarines to its fleet, such as the USS Massachusetts (SSN-798). Meanwhile, world powers are also working on autonomous submarines, which require no crew and can technically stay underwater for longer periods of time.
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