Diesel-electric multiple unit
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A Diesel-electric multiple unit (DEMU) is a type of railway vehicle. It is one variant of the diesel multiple unit (DMU). A DEMU consists of one or more railway cars, with at least one car containing a diesel generator and electric motors.
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[edit] Description
A DEMU is a combination of DMU and EMU technologies. They are effectively EMUs which have power supplied by an onboard generator and diesel engine, rather than from external power sources such as overhead wires or electrified tracks.
In a DEMU, a diesel engine drives a generator which produces (usually) AC current. This current can then be fed to traction motors in the wheels or bogies (not necessarily all on the car or train have motors). Power is controlled by the engine, the more fuel supplied to the engine, the faster the engine and generator spin and so the more current is generated for the motors.
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 and obtain electrical current from other cars in the consist which have a generator and engine.
Some DEMUs have the ability to use external electrical power sources where available, usually in areas where the pollution caused by internal combustion is not acceptable or simply as it is more economical. Others feature rheostatic braking (sometimes called dynamic, electric or regenerative braking), which is accomplished by switching the motors to act as generators that convert motion into electricity instead of electricity into motion. The energy produced is usually discarded as heat via resistors 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 principle reason for its use is to reduce wear and tear on the main service brakes (as seen on Bombardier's Class 220, 221 and 222 units).
[edit] History
DEMUs were widely used in the United Kingdom after their introduction in the 1960s by British Rail. At that time there was an urgent need to move away from steam which led to many experimental designs using Diesel-electric traction and multiple units.
A type of DEMU nick-named "Thumpers" (after their characteristic sound) was widely used in south-eastern England. They were intended as a stop-gap to cover lines that were shortly to be electrified. However it was about 40 years before they were withdrawn in 2004 to be replaced by more modern DMUs. The "Thumpers" were considered by many passengers to be uncomfortable, noisy and lacking in power, so they attracted a fan-club (that is a peculiarly British way of thinking).
Similar to the "Thumpers" were the "Hastings" DEMUs, introduced in 1957 for the London – Hastings service via Tunbridge Wells - see the Hastings Line.
The British Rail High Speed Train (HST) was a much more powerful version, branded as "InterCity 125" (because their top speed was 125 miles per hour). These trains have always run with 2 power cars and between 5 and 9 intermediate coaches. Although originally classified as DEMUs, the power cars later became Class 43 (the second use of the designation, the first being the "Warship" class) to allow flexibility in the event of failures, instead of being confined to the semi-permanent formations. Essentially the power cars are just special Diesel-Electric locomotives and the coaches modified from loco-hauled stock.
[edit] DEMUs in Britain today
DEMUs are now making a reappearance on British railways in the form of Class 220 Voyager, 221 Super-Voyager and 222 Meridian/Pioneer units, running many services previously provided by the High Speed Train and some faster DMUs (such as the Class 158, 166 168 and 170 series). These units are a sharp contrast to the "Thumpers" and "Hastings" of old, they are air-conditioned through out, with powered doors and a top speed of 125 mph. Acceleration is regarded as fast in comparison to the High Speed Train and DMUs they replace. They can reach 60mph in less than 60 seconds under good conditions. It is for this reason that Midland Mainline chose the type to replace their Class 170 Turbostars and Virgin Trains chose them to replace their HSTs. The Class 221 Super-Voyager is so advanced it even tilts where the infrastructure permits, letting it achieve higher speeds on sharp bends. The noise and vibration levels from the 750hp engines under the floors have made them unpopular with some people, in partiucular due to the long distances they often run (including the longest train journey in the UK, from Penzance to Aberdeen). Many feel that locomotive haulage or separate powercars (at least for engines, even if motors are still under passenger coaches) provides a superior experience on long distance journeys.
[edit] DEMUs in other countries
- Transwa Prospector (Australia)
- ICE-TD (Germany)
- Class 110 (India) (Ref: http://members.rediff.com/demucar/)
[edit] Driving a DEMU
Driving a modern DEMU is much like driving any other modern diesel train. The Bombardier Class 220/221/222 series has no extra gauges created by the use of an electrical transmission and the cab feels similar to other contemporary British DMUs (Pacer, Sprinter, Turbo, Clubman and Turbostar).
The controls vary a little, generally having more positions to cope with the higher speeds. Unlike the afore-mentioned DMUs where the drivers applies less than 50% power to begin moving the unit, in a DEMU the driver simply applies full power, this being one of the few differences between the two types and making it more similar to an EMU in this one respect. In the case of Class 22x there is an additional gauge showing total braking power applied by both the rheostatic and air braking.
[edit] Preservation
A number of older DEMU types (Thumper and Hasting units) have been preserved and are in use on private railways and for occasional main-line excursions. Notable examples are:
- Class 205 "one end Hampshire and one end Berkshire" DEMU 1118 located at pontypool and blaenavon railway
- Class 201 "Hastings" DEMU 1001 owned by Hasting Diesels Limited
- Class 205 "Hampshire" DEMU 1125 on the Watercress Line
- Class 207 "Oxted" DEMU 1317 on the Spa Valley Railway
