Editing Diesel multiple unit (section)

==Types by transmission==
DMUs are usually classified by the method of transmitting motive power to their wheels.{{Citation needed|date=October 2010}}

===Diesel–mechanical {{Anchor|DMMU}}===
In a diesel–mechanical multiple unit (DMMU), the rotating energy of the engine is transmitted via a [[Transmission (mechanics)|gearbox]] and driveshaft directly to the wheels of the train, like a [[automobile|car]]. The transmissions can be shifted manually by the driver, as in the great majority of first-generation [[British Rail]] DMUs, but in most applications, gears are changed automatically.

===Diesel–hydraulic===
[[File:JR shikoku for 2706 SA6D140HE-2 at tokushima.jpg|thumb|right|One of the underfloor diesel engines with hydraulic transmission under a car of [[JR Shikoku 2700 series]] DMU]]
In a diesel–hydraulic multiple unit (DHMU), a hydraulic [[torque converter]], a type of fluid coupling, acts as the transmission medium for the motive power of the diesel engine to turn the wheels. Some units feature a hybrid mix of hydraulic and mechanical transmissions, usually reverting to the latter at higher operating speeds as this decreases engine RPM and noise.

===Diesel–electric {{Anchor|DEMU}}===
[[File:Stadler Flirt DMU 2404 power car.jpg|thumb|right|"Power Pack" car of [[Stadler FLIRT]] DMU with open bay of a diesel-generator. The Jacobs bogies under this car do not have their own traction motors, which are mounted on bogies under other cars.]]
In a diesel–electric multiple unit (DEMU), a [[diesel engine]] drives an [[electrical generator]] or an [[Alternator#Diesel electric locomotive alternators|alternator]] which produces [[electrical energy]]. The generated current is then fed to electric [[traction motor]]s on the [[wheel]]s or [[bogie]]s in the same way as a conventional [[diesel–electric locomotive]].<ref name="gaz-voyager" />

On some DEMUs, such as the [[Bombardier Voyager]], each car is entirely self-contained and has its own engine, generator and electric motors.<ref name="gaz-voyager">{{cite web|url=http://www.railwaygazette.com/news/single-view/view//cutting-noise-and-smoothing-the-ride.html|title=Cutting noise and smoothing the ride|work=[[Railway Gazette International]]|date=1 August 2000|quote=In the Voyager application, every car has a Cummins underfloor engine and alternator supplying power to a pair of body-mounted traction motors. Each drives one inner axle through a cardan shaft and axle-mounted final drive gearbox.|access-date=20 January 2011|url-status=dead|archive-url=https://web.archive.org/web/20120604022558/http://www.railwaygazette.com/news/single-view/view/cutting-noise-and-smoothing-the-ride.html|archive-date=4 June 2012}}</ref> In other designs, such as the [[British Rail Class 207#Technical details|British Rail Class 207]] or the [[Stadler GTW]] and [[Stadler Flirt|Stadler FLIRT DMU]],<ref>Stadler Flirt DMU/DEMU example: [[commons:Category:Elektriraudtee Classes 2200, 2300 and 2400|Elron Elektriraudtee Classes 2200, 2300 and 2400]], [[commons:File:TEXRail Stadler FLIRT DFW Airport Nov 2019.jpg|TEXRail Stadler FLIRT]]</ref> some cars within the [[consist]] may be entirely unpowered or only feature electric motors, obtaining electric current from other cars in the consist which have a generator and engine.

With diesel–electric transmission, some DMU can be no other than an [[Electric multiple unit|EMU]] without [[Pantograph (transport)|pantograph]] or [[contact shoe]]s (for use on the former [[British Rail]] [[Southern Region of British Railways|Southern Region]]), which "is substituted" by one or more on-board [[diesel generator]]s; this kind of DEMU can be potentially upgraded to [[electro-diesel multiple unit]] (EDMU), becoming a bi-mode multiple units train (BMU), just adding one or two pantographs or contact shoes (with opportune converters,{{clarify|date=August 2022}} if necessary) and related modifications on the electric system.{{citation needed|date=August 2022}}