How do diesel subs submerge? It's been over 100 years since subs similar to today's were used in war... Surprisingly, a lot of people don't seem to know how they navigate while submerged. Diesel subs get their propulsion from diesel engines, aka internal combustion engines. But... whether it's internal or external combustion, an engine is something that burns fuel to turn heat energy into kinetic energy. To be more specific, it burns fuel to expand heated air, and that expansion force moves the pistons.

Combustion needs air (oxygen), and the air needs to expand to create kinetic energy. As a byproduct, you get exhaust. But there's no air underwater, right? So how do they burn fuel and expand air? Even if you brought oxygen tanks underwater to burn fuel... if the exhaust stays inside the sealed sub, the crew would all suffocate. If you vent it outside... do they just sail along farting out bubbles? Then the bubbles would be visible, and they'd get caught. The answer lies in cars.

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That gear you see on off-road SUVs is called a snorkel.

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Even while driving through water like this, they can suck air into the intake to run the engine.

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It's the same principle as the snorkels divers use. It looks similar, too. If you put this on a sub, it sucks in air (intake) to burn fuel and run the engine, and vents the exhaust (exhaust).

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Of course, if you submerge with the intake and exhaust sticking out, people can see the spray from miles away... and you'll get caught immediately.

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The solution to this also lies in cars. There are cars that use the engine's power to run but charge a battery with the leftover energy, then use that battery to run a motor every now and then to save fuel. That's the 'hybrid' car that has become mainstream today.

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The world's first 'successful mass-produced hybrid' was the Toyota Prius. While that came out in 1997, the idea of using an engine to charge a battery and then using that battery to run a motor was around since the late 19th century. Of course, batteries back then had garbage efficiency and were ridiculously huge, so they couldn't be used in cars, but they could be used in subs (actually, this was the only way to submerge without a snorkel...). In other words, diesel subs sail on the surface or use a snorkel in safe zones, but in high-risk areas, they dive deeper and run on the batteries they charged earlier. That's why diesel subs have limits on how long they can stay under. On the other hand, nuclear subs use heat from nuclear fission to boil water, creating steam that turns a turbine to generate electricity... Since it doesn't burn oxygen and water can be reused after cooling, there's no loss and no need for intake/exhaust. You can even split water with electricity to get oxygen, so as long as there's food, they can stay submerged forever.