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A number of vehicles use a diesel-electric powerplant for providing locomotion. A diesel-electric powerplant includes a diesel engine connected to an electrical generator, creating electricity that powers electric traction motors.
This kind of power transmission is used by locomotives (see that article for details), used for pulling or pushing trains. Diesel-electric powerplants have also been used in submarines and surface ships and some land vehicles. Vehicles using diesel-electric power system can be considered as a class of hybrid electric vehicle, especially, if the electric energy is also coming from rechargeable batteries.
Submarines
When the diesel engine was first installed in submarines before the First World War, it revolutionized submarine tactics because its range (or operating time) far surpassed that of the gasoline engine, and leaked vapors of its fuel were far less prone to explode within a submarine cabin. The fuel was also easier to stow. In submarines, the engine is connected to generators to produce electric energy that is stored in batteries for running underwater where oxygen is not available to feed the engines. It had been standard for them to use a driveshaft directly connecting the diesel engine, generator and propellers. There were a variety of clutch mechanisms for various connections as well. However, the long driveshaft has often been a source of trouble, also producing noise and vibration.
Some Soviet submarines had three propellers, and could be used in a variety of ways: each running on its diesel engine; either the central one or the outer two could each be connected to a diesel engine with or without the other propeller running on electric; or the engines could via snorkel be recharging the batteries while the central propeller was quietly running on its electric motor; or all three could be running on electric motors.
The performance of its diesel engine was critical for a conventional submarine, and not easy to ensure. Navies imported and copied successful designs. Although the Germans had some excellent diesels for their U-boats, they also had some which were not so good. The Americans had a similar history; their last diesel engine design was radial in form, its driveshaft axis angled vertically. To replace this unreliable but space-efficient engine, submarines had to be sawn in half through the engine room and proven engines installed in a lengthened hull section. Since then, European submarine diesels have shown advances.
Modern diesel-electric submarines don't have a direct connection of the diesel engine to the propeller anymore: The usually single propeller is driven directly by an electric motor. Two or more diesel-generators provide electric energy for loading the batteries and/or driving the electric motor. This allows to mechanically insulate the noisy engine compartement from the outer pressure hull. reducing the acoustic signature of the submarine. Even some nuclear submarines decouple their reactor room this way, e.g. all French classes, like the Rubis and Redoutable and the Chinese Type 093 class, have turbo-electric propulsion.
Ships
Even though diesel-powered motor ships have been in use since 1912 (e.g. with the Selandia) and steam-turbine/electric propulsion at least since the 1920s (Tennessee class battleships), the practice of using diesel-electric powerplants in surface ships has been a more recent development. The Finnish "coastal battleship " Ilmarinen, laid down in 1929, was among the first surface ships to use diesel-electric transmission. Later the technology was used in diesel powered icebreakers.
Some modern ships, including cruise ships and icebreakers, use electric motors in pods called azimuth thrusters underneath to allow for 360° rotation, meaning that the ships are far more maneuverable.
Some vehicles also use gas turbines in the same way. In fact, some use a combination: the Queen Mary 2 has a set of diesel engines in the bottom of the ship plus a gas turbine near the top exhaust tower. All are used for generating electrical power, and none of the ship's propellers are directly connected to any engines.
Locomotives
In the 1920s, diesel-electric technology first saw limited use in switchers (or shunters), locomotives used for moving trains around in railroad yards and assembling and disassembling them. One of the first companies to offer "Oil-Electric" locomotives was the American Locomotive Company. The ALCO HH series of diesel-electric switcher entered series production in 1931. In the 1930s, the system was adapted for streamliners, the fastest trains of their day. Diesel-electric powerplants became popular because they greatly simplified the way motive power was transmitted to the wheels and because they were both more efficient and had greatly reduced maintenance requirements. Direct-drive transmissions can become very complex, considering that a typical locomotive has four or more axles. Some attempts were made at using hydraulic fluid as a transmission medium, and it proved to be somewhat more efficient than diesel-electric technology. However, the complexity of both the direct-drive and hydraulic systems meant that breakdowns were more common.
Other land vehicles
Diesel-electric propulsion was tried on some military vehicles, such as tanks. One example was the ill-fated Maus tank. Currently no tank uses this principle for movement, but it's quite common to train the turret and/or guns with electric motors powered by diesel or turbine APUs.
More success had diesel-electric propulsion on the civilian market: Big mining machines like the Liebherr T282B dump truck or LeTourneau L-2350 wheel loader are powered that way. Also NASA's huge Crawler-Transporters are propelled diesel-electrically.
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"Diesel-electric"