A dieselâÂÂelectric transmission, or dieselâÂÂelectric powertrain, is a transmission system powered by diesel engines which generate electricity to power electric motors on vehicles in road, rail, and marine transport. DieselâÂÂelectric transmission is similar to petrolâÂÂelectric transmission, which are powered by petrol engines.
DieselâÂÂelectric transmission is used on railways by dieselâÂÂelectric locomotives and dieselâÂÂelectric multiple units, as electric motors are able to supply full torque from 0 RPM. DieselâÂÂelectric systems are also used in marine transport, including submarines, and on some other land vehicles.
A major advantage of dieselâÂÂelectric transmission is that it avoids the need for a gearbox, by converting the mechanical force of the diesel engine into electrical energy (through an alternator), 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. Similar arrangements with other sources of power are petrolâÂÂelectric transmission (powered by petrol/gasoline engine), and turbineâÂÂelectric powertrain, used with gas turbines.
The gearbox required for a powerful diesel engine directly driving more than one output (e.g., multiple axles) can be very complex and potentially a point of failure; dieselâÂÂelectric transmission does away with the need for a gearbox. The absence of a gearbox also eliminates the need for gear changes, avoids uneven acceleration caused by the disengagement of a clutch. With auxiliary batteries the motors can be driven without the engine running constantly, for example in a clean-air zone where use of an internal-combustion engine is restricted.
The first diesel motorship was also the first dieselâÂÂelectric ship, the Russian tanker Vandal from Branobel, which was launched in 1903. Steam turbineâÂÂelectric propulsion has been in use since the 1920s (s), using dieselâÂÂelectric powerplants in surface ships has increased lately. The Finnish coastal defence ships Ilmarinen and Väinämöinen laid down in 1928âÂÂ1929, were among the first surface ships to use dieselâÂÂelectric transmission. Later, the technology was used in diesel powered icebreakers.
In World War II, the United States Navy built dieselâÂÂelectric surface warships. Due to machinery shortages destroyer escorts of the and es were dieselâÂÂelectric, with half their designed power (The and es were full-power steam turbineâÂÂelectric). The s, on the other hand, were designed for dieselâÂÂelectric propulsion because of its flexibility and resistance to damage.
Some modern dieselâÂÂelectric ships, including cruise ships and icebreakers, use electric motors in pods called azimuth thrusters underneath to allow for 360ð 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. On the surface, the diesel engine propelled the boat and was also used as a generator to recharge the batteries and supply other electric loads. The engine was disconnected for submerged operation, with batteries powering the electric motor and electrical equipment.
In a true dieselâÂÂelectric 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 has some disadvantages compared to using the diesel engine to drive the propeller, 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, protecting the submarine from detection by reducing its acoustic signature when surfaced. Some nuclear submarines also use a similar turboâÂÂelectric propulsion system, with propulsion turbo generators driven by reactor plant steam.
Among the pioneering users of true dieselâÂÂelectric transmission was the Swedish Navy with its first submarine, (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 (, , and ), all using dieselâÂÂelectric transmission. While Sweden temporarily abandoned dieselâÂÂelectric 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, dieselâÂÂelectric transmission has been consistently used for all new classes of Swedish submarines, albeit supplemented by air-independent propulsion (AIP) provided by Stirling engines, beginning with HMS Näcken in 1988.
Another early adopter of dieselâÂÂelectric 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 , , and , then put into production for 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 dieselâÂÂelectric 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 dieselâÂÂelectric 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 petrolâÂÂelectric 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.
In the 1920s, dieselâÂÂelectric technology first saw limited use in switcher locomotives (UK: shunter locomotives), locomotives used for moving trains around in railroad yards and assembling and disassembling them. An early company offering "OilâÂÂElectric" locomotives was the American Locomotive Company (ALCO). The ALCO HH series of dieselâÂÂelectric switcher entered series production in 1931. In the 1930s, the system was adapted for streamliners, the fastest trains of their day. DieselâÂÂelectric 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. A typical large diesel engine can generate 4,000 horsepower or more, and cannot run at more than about 2,100 rpm; it would need 20 or 30 gears to travel at speeds from slow up to even . A gearbox to handle these gears at that power, and furthermore to drive four or more axles, would be huge, complicated, inefficient, and subject to mechanical failure. Coupling the diesel to a generator eliminates this problem. An alternative is to use a torque converter or fluid coupling to replace the gearbox in a direct drive system.
DieselâÂÂelectric powered 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 dieselâÂÂelectric 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 and Nova Bus are major customer for the BAE HybriDrive system. Mercedes-Benz makes their own dieselâÂÂelectric drive system, which is used in their Citaro. The only bus that runs on single dieselâÂÂelectric transmission is the Mercedes Benz Cito low floor concept bus which was introduced in 1998.
Examples include:
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.
DieselâÂÂelectric propulsion has been tried on some military vehicles, such as tanks. The German armored vehicles VK 45.01 (P), Elefant, and Panzer VIII Maus of the Second World War were petrolâÂÂelectric or dieselâÂÂelectric propelled. 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 dieselâÂÂelectric drives to improve fuel efficiency while reducing the size, weight and noise of the power plant. Attempts with dieselâÂÂelectric drives on wheeled military vehicles include the unsuccessful ACEC Cobra, MGV, and XM1219 armed robotic vehicle.