page_title: Diesel locomotive
section_title: Diesel–mechanical
hierarchical_section_title: Diesel locomotive / Transmission types / Diesel–mechanical
caption_attribution_description: BR Class 03, no. D2069 at Doncaster Works on 27th July 2003.
context_page_description: A diesel locomotive is a type of railway locomotive in which the prime mover is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is conveyed to the driving wheels.
  Early internal combustion locomotives and railcars used kerosene and gasoline as their fuel. Rudolf Diesel patented his first compression-ignition engine in 1898, and steady improvements to the design of diesel engines reduced their physical size and improved their power-to-weight ratios to a point where one could be mounted in a locomotive. Internal combustion engines only operate efficiently within a limited torque range, and while low power gasoline engines could be coupled to mechanical transmissions, the more powerful diesel engines required the development of new forms of transmission. This is because clutches would need to be very large at these power levels and would not fit in a standard 2.5 m-wide locomotive frame, or wear too quickly to be useful.
  The first successful diesel engines used diesel–electric transmissions, and by 1925 a small number of diesel locomotives of 600 hp were in service in the United States.
context_section_description: A diesel–mechanical locomotive uses a mechanical transmission in a fashion similar to that employed in most road vehicles. This type of transmission is generally limited to low-powered, low speed shunting (switching) locomotives, lightweight multiple units and self-propelled railcars.
  The mechanical transmissions used for railroad propulsion are generally more complex and much more robust than standard-road versions.  There is usually a fluid coupling interposed between the engine and gearbox, and the gearbox is often of the epicyclic (planetary) type to permit shifting while under load.  Various systems have been devised to minimise the break in transmission during gear changing; e.g., the S.S.S. (synchro-self-shifting) gearbox used by Hudswell Clarke.
  Diesel–mechanical propulsion is limited by the difficulty of building a reasonably sized transmission capable of coping with the power and torque required to move a heavy train.  A number of attempts to use diesel–mechanical propulsion in high power applications have been made (e.g., the 1,500 kW (2,000 hp) British Rail 10100 locomotive), although none have proved successful in the end.
caption: A British Rail Class 03 diesel–mechanical shunter with a jackshaft under the cab.
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