Diesel multiple unit
From JNSwiki
A diesel multiple unit or DMU is a multiple unit train consisting of multiple carriages powered by one or more on-board diesel engines. They may also be referred to as a railcar.
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Design
The diesel engine may be located above the frame in an engine bay or under the floor. Driving controls can be at both ends, on one end or none on the center of a three car sets normally called a "trailer cars", these trailer cars may have motors that rely on the cab cars for control.
Types
DMUs are usually classified by the method of transmitting motive power to their wheels.
Diesel-mechanical
In a diesel-mechanical multiple unit (DMMU) the rotating energy of the engine is transmitted via a Transmission |gearbox and driveshaft directly to the wheels of the train, much like a automobile|car. The transmissions can be shifted manually by the driver, as in the great majority of first-generation DMUs, but in most applications gears are changed automatically.
Diesel-hydraulic
In a diesel-hydraulic multiple unit, 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
In a diesel-electric multiple unit (DEMU) a diesel engine drives an electrical generator which produces electrical energy. The generated current is then fed to electric traction motors on the wheels or bogies in the same way as a conventional Diesel-electric locomotive.
In most modern DEMUs, each car is entirely self-contained and has its own engine, generator and electric motors. In older designs some cars within the consist may be entirely unpowered or only feature electric motors, obtaining electrical current from other cars in the consist which have a generator and engine.
Direct-drive diesel locomotives often require an impractical number of gears to keep the engine within its powerband; coupling the diesel to a generator eliminates this problem. Power still needs to be transmitted to the generator or alternator via a simple gearbox but this is advantageous because;
- Mere cables transmit the power to the axle traction motors, not a complex system of rods
- Sending power to a generator via a low powerband, i.e., diesel, is analogous to sending power to a propeller, in that one gear ratio is enough, unlike sending power to wheels, where several ratios are necessary. This is because the high friction between wheels and contact surface requires a low gear for takeoff to avoid stalling, and higher gears thereafter, since the output shaft moves relative to vehicle speed and would over-rev the engine if just one gear was available. In a generator or alternator, the force is magnetism, not a high friction surface so a high gear is fine since the magnetism is not great enough to stall the engine. The result is that the diesel-electric system puts the diesel’s low powerband to best use.
- Rheostatic braking, sometimes called dynamic, electric or regenerative braking, is also available on some DEMUs. By attaching the motors to an electrical load they act as generators and slow the train down. The load is usually resistors which dissipate electrical energy as heat to the atmosphere, though it is sometimes used to heat the interiors. In some cases it is stored for later use or passed back into external power supplies. The advantage for this method of braking is reduced wear and tear on the main service brakes.
Benefits
A train composed of DMU cars scales well as it allows extra passenger capacity to be added at the same time as motive power. It also permits passenger capacity to be matched to demand, and for trains to be split and joined en-route. It is not necessary to match the power available to the size and weight of the train- each unit is capable of moving itself, so as units are added, the power available to move the train increases by the necessary amount.
Distribution of the propulsion among the cars also results in a system that is less vulnerable to single-point-of-failure outages. Many classes of DMU are capable of operating with faulty units still in the consist. Because of the self contained nature of diesel engines, there is no need to run overhead electric lines or third rail electrified track, which can result in lower system construction costs.
These advantages outweigh the underfloor noise and vibration that sometimes affects these type of train.
The development of DMUs in Japan started in 1950s following the improvement of fuel supply that was critical during World War II. In 1953, the Japanese National Railways put the hydraulic torque converter into practical use. This invention facilitated the development of DMUs, which spread all over the nation as not only on local services but also on long distance express services. In 1960, the 9-car KiHa 80 series DMUs debuted on Hatsukari limited express service connecting Tokyo and Aomori (about 750 km).
Presently, all the passenger railway companies of Japan Railways Group (JR Group) operate DMUs on semi-trunk and local lines while almost all of trunk lines have been electrified. On the JR lines not yet electrified, locomotive trains were abandoned but only small number of sleeper trains. There are also a number of independent railway lines that operate DMUs.
