Diesel engines are used to fuel diesel locomotives. In the earlier times of development and improvement of diesel engines, numerous transmission systems with different performance were used, the electric transmission being the most outstanding among all.
Among all the varieties of diesel trains, a method was developed by which mechanical power was transmitted to the driving wheels of the locomotive.
When, after the world war, the world cured itself monetarily, it did so by a wide selection of diesel trains in different countries. Diesel locomotives gave tremendous performance and flexibility, and proved to be better than steam locomotives, as well as requiring much less maintenance and operating costs. Diesel-hydraulic were introduced in the mid-20th century, but after the 1970s diesel-electric transmissions were used at a higher level.
A motorized transmission to transmit power to all wheels is used in diesel locomotives. This type of transmission is usually limited to low-speed, low-power shunting locomotives, self-propelled cars, and numerous light units. The first diesel locomotives were diesel-mechanized. Most diesel locomotives today are diesel-electric locomotives.
The most important and absolutely vital factors of a diesel-electric propulsion system are the diesel engines (also called the traction unit), the central generator/alternator-rectifier, the control system consisting of the engine regulator and electrical or electronic elements, the traction motors (usually with four or six axes) including rectifiers, switchgear and other elements that regulate or modify the electrical supply to the traction motors.
In the most general case, the generator may be directly connected to the engines with only a very simple switchgear. In most cases, the generator is only connected to the engines with edge switchgear.
Diesel locomotives with hydraulic transmission are called diesel-hydraulic locomotives. In this configuration, they use more than one torque converter combined with gears, with a mechanical final drive gear to transmit power from the diesel engine to the wheels.