Diesel engines need air, submarines run under water, so how do diesel submarines work? – Jalopnik

Although diesel engines do not require spark plugs, they do require air ventilation for proper combustion. This is a problem for ships that sail underwater, such as submarines. Yet diesel submarines exist and are used all over the world. How is that even possible? Well, there is a difference in how energy is generated in a submarine and how that energy is actually used.

Diesel submarines are equipped with one or more diesel engines, electric motors and a large battery bank. The diesel engines are not used to propel the submarine when it is completely submerged. Instead, they only work when the submarine remains on or just below the surface of the water. The engines essentially just power generators to charge the submarine’s batteries and provide propulsion as the ship surfaces.

Once the submarine dives, the diesel engines shut down completely and all movements depend on the electric motors powered by the stored electrical energy.

Air can enter a submarine in two ways: by surfacing, or by operating in shallow water with a snorkel. This snorkel is a pipe that extends just above the surface and sucks air into the engine, which aids combustion. It is also used to expel exhaust gases that are pushed back into the atmosphere.

Now, snorkeling allows the submarine to recharge its batteries while the hull remains hidden, but that comes at a price. When snorkeling, the submarine must follow a number of rules and deal with increased vulnerability. For example, the submarine must move slowly. This is because snorkel masts are fragile and snorkeling can blow up the submarine’s disguise. All of this limits how long the submarine can stay above the surface before completely submerging.

Therefore, while snorkeling, diesel engines perform their most important task, which is to replenish electrical energy. This is achieved by using large generators that convert mechanical energy from the engine into electricity, recharging the batteries that have been discharged during the immersion.

A diesel submarine essentially turns into an electric vessel when it submerges. Propulsion is provided by propeller shafts, which in turn are driven by battery-powered electric motors. Both the engines and motors are mechanically isolated. This design choice improves the reliability of both components because vibration transfer from one to the other is eliminated. It also allows the engines and propellers to run at speeds appropriate for their respective tasks.

Because a battery can only store a limited amount of energy, underwater operations are a constant trade-off between time and speed. The movement speed determines how quickly the batteries run out. A submarine that can travel at low speeds for days may also struggle to stay submerged for only a few hours at high speeds.

Commanding officers must keep these tradeoffs in mind when planning missions. Distance from friendly bases is another important consideration, as is the ability to surface or snorkel without attracting attention.

Diesel engines require a constant airflow to run smoothly. This is because the design of the two-stroke engine (which GM plans to reintroduce into cars, by the way) relies on a process called “scavenging,” which simultaneously refills the cylinders and expel exhaust gases.

Fresh air is forced into the cylinder using fans, which push exhaust gases out through open ports or valves. It’s a pretty cool concept and can also help you increase your car’s horsepower without a complete engine swap. This fresh air can also be taken from the interior spaces of the submarine, then filtered, silenced and finally pressurized before entering the engine. All excess air is used to ensure complete combustion and removal of exhaust gases. The burned gases are passed through valves, pipes and mufflers before being completely vented outside the hull. All these systems are water cooled and designed to operate quietly.

When the submarines operate underwater in snorkel mode, they have to push the exhaust gases against the water pressure. This requires additional outlet pressure to prevent seawater from entering the system.

In addition to having to surface to charge batteries, release exhaust fumes, and draw in oxygen, submarines also need to surface for communications, as radio signals do not travel effectively through deep water – this is why even nuclear submarines surface for short periods of time.

Then there is the human element. Between food, medical care and crew changes, logistics add up quickly – autonomous submarines can alleviate this problem, and Boeing has been quietly developing them. But in addition to biological needs, maintenance support is also crucial. This is because mechanical systems wear out over time and some cannot be repaired while submerged.

Regulations stipulate that the crew must be able to survive for at least six days if surfacing becomes impossible. This in itself reinforces the need for regular resurfacing intervals. However, surfacing comes with risks, which is why diesel submarines limit how often and how long they stay on the surface. Therefore, an all-hands-on-deck approach to supporting infrastructure is necessary when it comes to planning and coordination.

Thanks to significant improvements in engine efficiency and air-independent propulsion systems, diesel submarines can now stay submerged for much longer than before, while producing much less noise.

In fact, modern diesel submarines make almost no noise when operating on their batteries, apart from the subtle noise of mechanical components and the rushing of water around the hull. The silent operation is a major advantage in coastal operations and generally in areas close to the coastline, where the submarine does not have to travel far across open water to reach safety.

Diesel submarines work because they are designed around these inherent limitations. They trade unlimited endurance for cost control and stealth at low speeds. The result is a system that works effectively by separating energy generation from underwater movement, using air only when necessary and electricity when the success of the operation depends on maintaining silence.