A diesel-electric transmission, or diesel-electric powertrain is a transmission system for vehicles powered by diesel engines in road, rail, and marine transport. Diesel-electric transmission is based on petrol-electric transmission, a very similar transmission system used for petrol engines.
Diesel-electric transmission is used on railways by diesel-electric locomotives and diesel-electric multiple units, as electric motors are able to supply full torque at 0 RPM. Diesel-electric systems are also used in marine transport, including submarines, and on some land vehicles.
The defining characteristic of diesel-electric transmission is that it avoids the need for a gearbox, by converting the mechanical force of the diesel engine into electrical energy (through a dynamo), and using the electrical energy to drive traction motors, which propel the vehicle mechanically. The traction motors may be powered directly or via rechargeable batteries, making the vehicle a type of hybrid electric vehicle. This method of transmission is sometimes termed electric transmission, as it is identical to petrol-electric transmission, which is used on vehicles powered by petrol engines, and to turbine-electric transmission, which is used for gas turbines.
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The absence of a gearbox offers several advantages, as it removes the need for gear changes, thus eliminating the unevenness of acceleration caused by the disengagement of a clutch.
The first diesel motorship was also the first diesel-electric ship, the Russian tanker Vandal from Branobel, which was launched in 1903. Steam turbine-electric propulsion has been in use since the 1920s ( Tennessee-class battleships), using diesel-electric powerplants in surface ships has increased lately. The Finnish coastal defence ships Ilmarinen and Väinämöinen laid down in 1928-1929, were among the first surface ships to use diesel-electric transmission. Later, the technology was used in diesel powered icebreakers.
In World War II, the United States Navy built diesel-electric surface warships. Due to machinery shortages destroyer escorts of the Evarts and Cannon classes were diesel-electric, with half their designed horsepower (The Buckley and Rudderow classes were full-power steam turbine-electric). The Wind-class icebreakers, on the other hand, were designed for diesel-electric propulsion because of its flexibility and resistance to damage.
Some modern diesel-electric ships, including cruise ships and icebreakers, use electric motors in pods called azimuth thrusters underneath to allow for 360° rotation, making the ships far more maneuverable. An example of this is Symphony of the Seas, the largest passenger ship as of 2019.
Gas turbines are also used for electrical power generation and some ships use a combination: Queen Mary 2 has a set of diesel engines in the bottom of the ship plus two gas turbines mounted near the main funnel; all are used for generating electrical power, including those used to drive the propellers. This provides a relatively simple way to use the high-speed, low-torque output of a turbine to drive a low-speed propeller, without the need for excessive reduction gearing.
Most early submarines used a direct mechanical connection between the combustion engine and propeller, switching between diesel engines for surface running and electric motors for submerged propulsion. This was effectively a "parallel" type of hybrid, since the motor and engine were coupled to the same shaft. On the surface, the motor (driven by the engine) was used as a generator to recharge the batteries and supply other electric loads. The engine would be disconnected for submerged operation, with batteries powering the electric motor and supplying all other power as well.
In a true diesel-electric transmission arrangement, by contrast, the propeller or propellers are always driven directly or through reduction gears by one or more electric motors, while one or more diesel generators provide electric energy for charging the batteries and driving the motors. While this solution comes with disadvantages as well as advantages in comparison with the direct mechanical connection between the diesel engine and the propeller that was initially the most common arrangement, the advantages were eventually found to be more important. One of several significant advantages is that it mechanically isolates the noisy engine compartment from the outer pressure hull and reduces the acoustic signature of the submarine when surfaced. Some nuclear submarines also use a similar turbo-electric propulsion system, with propulsion turbo generators driven by reactor plant steam.
Among the pioneering users of true diesel-electric transmission was the Swedish Navy with its first submarine, HMS Hajen (later renamed Ub no 1), launched in 1904 and originally equipped with a semi-diesel engine (a hot-bulb engine primarily meant to be fueled by kerosene), later replaced by a true diesel. From 1909 to 1916, the Swedish Navy launched another seven submarines in three different classes (2nd class, Laxen class, and Braxen class), all using diesel-electric transmission. While Sweden temporarily abandoned diesel-electric transmission as it started to buy submarine designs from abroad in the mid 1910s, the technology was immediately reintroduced when Sweden began to design its own submarines again in the mid 1930s. From that point onwards, diesel-electric transmission has been consistently used for all new classes of Swedish submarines, albeit supplemented by air-independent propulsion (AIP) as provided by Stirling engines beginning with HMS Näcken in 1988.
Another early adopter of diesel-electric transmission was the United States Navy, whose Bureau of Steam Engineering proposed its use in 1928. It was subsequently tried in the S-class submarines S-3, S-6, and S-7 before being put into production with the Porpoise class of the 1930s. From that point onwards, it continued to be used on most US conventional submarines.
Apart from the British U-class and some submarines of the Imperial Japanese Navy that used separate diesel generators for low speed running, few navies other than those of Sweden and the US made much use of diesel-electric transmission before 1945. After World War II, by contrast, it gradually became the dominant mode of propulsion for conventional submarines. However, its adoption was not always swift. Notably, the Soviet Navy did not introduce diesel-electric transmission on its conventional submarines until 1980 with its Paltus class.
During World War I, there was a strategic need for rail engines without plumes of smoke above them. Diesel technology was not yet sufficiently developed but a few precursor attempts were made, especially for petrol-electric transmissions by the French (Crochat-Collardeau, patent dated 1912 also used for tanks and trucks) and British (Dick, Kerr & Co and British Westinghouse). About 300 of these locomotives, only 96 being standard gauge, were in use at various points in the conflict. Even before the war, the GE 57-ton gas-electric boxcab had been produced in the USA.
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. An early company offering "Oil-Electric" locomotives was the American Locomotive Company (ALCO). 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. Additionally, a direct-drive diesel locomotive would require an impractical number of gears to keep the engine within its powerband; coupling the diesel to a generator eliminates this problem. An alternative is to use a torque converter or fluid coupling in a direct drive system to replace the gearbox. Hydraulic transmissions are claimed to be somewhat more efficient than diesel-electric technology.
Diesel electric based buses have also been produced, including hybrid systems able to run on and store electrical power in batteries. The two main providers of hybrid systems for diesel-electric transit buses include Allison Transmission and BAE Systems. New Flyer Industries, Gillig Corporation, and North American Bus Industries are major customers for the Allison EP hybrid systems, while Orion Bus Industries is a major customer for the BAE HybriDrive system. Mercedes-Benz makes their own diesel-electric drive system, which is used in their Citaro. The only bus that runs on single diesel-electric transmission is the Mercedes Benz Cito low floor concept bus which was introduced in 1998.
In the automobile industry, diesel engines in combination with electric transmissions and battery power are being developed for future vehicle drive systems. Partnership for a New Generation of Vehicles was a cooperative research program between the U.S. government and "The Big Three" automobile manufacturers (DaimlerChrysler, Ford and General Motors) that developed diesel hybrid cars.
Diesel-electric propulsion has been tried on some military vehicles, such as tanks. The prototype TOG1 and TOG2 super heavy tanks of the Second World War used twin generators driven by V12 diesel engines. More recent prototypes include the SEP modular armoured vehicle and T95e. Future tanks may use diesel-electric drives to improve fuel efficiency while reducing the size, weight and noise of the power plant. Attempts with diesel-electric drives on wheeled military vehicles include the unsuccessful ACEC Cobra, MGV, and XM1219 Armed Robotic Vehicle.