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 motors. The most well-known vehicle to use this technology is the locomotive (see that article for details), used for pulling or pushing trains. Diesel-electric powerplants have also been used in submarines and surface ships. Vehicles using a diesel-electric power system can be considered as a class of hybrid electric vehicle.
In the 1920s, diesel-electric technology first saw limited use in switchers, locomotives used for moving trains around in railroad yards and assembling and disassembling them. 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 more efficient. 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 breakdowns were more common.
Diesel engines have been used on submarines and ships for many years. It has been common for them to use a driveshaft directly connecting the engine and propellers. However, the long driveshaft has often been a source of trouble, also producing noise and vibration.
Diesel submarines include generators for storing power in batteries, used for running underwater where oxygen is not available to feed the engines. Most diesel subs have multiple engines (usually two), so one can be used to directly power a propeller, and the other can be used to run a generator.
The practice of using diesel-electric powerplants in surface ships has been a more recent development. Some use electric motors in pods 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.
Some military vehicles, such as tanks, also make use of the basic technology, whether driven by diesel or gas turbine engines.