Electric transmission (propulsion)
Electric transmission or electric powerchain is a transmission system for propelling motor vehicles. It is used for a variety of applications in road, rail, and marine transport.
The defining characteristic of electric transmission is that it avoids the need for a gearbox, by converting the mechanical force of the engine into electrical energy (through a dynamo or alternator), and using the electrical energy to drive traction motors, which propel the vehicle mechanically. The traction motors may be driven directly, or in some cases, via a rechargeable battery; if rechargeable batteries are used, the vehicle is considered to be a hybrid vehicle.
Electric transmission is usually classified as multiple separate transmission methods, according to engine type; for example, a vehicle powered by a petrol engine, using electrical transmission, would usually be classified as having petrol-electric transmission, despite the fact that the petrol engine is part of the engine, the transmission being a separate system.
Electric transmission was first used in the early 1920s. It was generally found to be unfavourable compared to the other methods available at the time, traditional steam locomotive transmission being up to 15% more efficient. However, by the late 1920s it was recognised as the best for larger vehicles.[1]
Advantages and disadvantages
The primary advantage of electric transmission is that it does not require gear changes, which allows for smoother acceleration. It can also offer substantial advantages,[1] as it reduces energy wastage which occurs in mechanical transmission, if the operator of the vehicle neglects to change the vehicle's gear. This is especially true in heavier vehicles,[1] where the force required to start moving is very high compared to that needed to maintain speed if already in motion; hence this system is most used by ships and railway locomotives; most cars instead use mechanical transmission, as they have lighter loads, and are more lightweight. However, these cost savings are somewhat offset by the increased weight, and higher initial cost of the generator, wiring, and motor.
Applications
- Petrol-electric transmission, for use with petrol engines. Substantially used in many industries in the early 20th century, but died out around the period of the Second World War due to the greater popularity of diesel engines. Use today in combination with energy stored in a battery pack as a hybrid electric vehicle
- Diesel-electric transmission, for use with diesel engines. Mostly superseded petrol-electric transmission, taking much of the same markets. Much-used in diesel-electric locomotives and diesel-electric multiple units, on railways. Used in some road vehicles such as buses and trucks in combination with a battery pack to store the energy needed to drive the electric motor used as a hybrid electric vehicle
- Turbo-electric transmission, for use with steam turbine engines. Substantially used by ships, especially military warships.
- Turbine-electric transmission, for use with gas turbine engines. Has mostly superseded turbo-electric transmission. Much used in ships, and occasionally gas turbine-electric locomotives, on railways.
References
- Duffy, Michael C. (2003). Electric Railways: 1880-1990. IET. pp. 163–164. ISBN 978-0-85296-805-5. Retrieved 23 May 2020.