Diesel-electric locomotive

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A locomotive (from lat. loco motivus) is a railway vehicle that provides the motive power for a train, and has no payload capacity of its own; its sole purpose is to move the train along the tracks. In contrast, many trains feature self-propelled payload-carrying vehicles; these are not normally considered locomotives, and may be referred to as multiple units or railcars; the use of these self-propelled vehicles is increasingly common for passenger trains, but very rare for freight (see however CargoSprinter). Vehicles which provide the motive power to haul an unpowered train, but are not generally considered locomotives because they have payload space or are rarely detached from their trains, are known as power cars.

Traditionally, locomotives haul (pull) their trains. Increasingly common these days in local passenger service is push-pull operation, where a locomotive pulls the train in one direction and pushes it in the other, and is therefore optionally controlled from a control cab at the opposite end of the train. This is especially true of "High Speed Rail lines", such as the Japans Shinkansen and Frances TGV trains.

Great Western Railway No. 6833 Calcot Grange, a 4-6-0 Grange class steam locomotive, at Bristol Temple Meads station, Bristol, England

Origins

The first successful locomotives were built by Cornish inventor Richard Trevithick. In 1804 his unnamed locomotive hauled a train along the tramway of the Penydarren ironworks, near Merthyr Tydfil in Wales. Although the locomotive hauled a train of 10 tons of iron and 70 passengers in five wagons over nine miles it was too heavy for the cast iron rails used at the time. The locomotive only ran three journeys before it was abandoned.

In 1813, George Stephenson persuaded the manager of the Killingworth colliery where he worked to allow him to build a steam-powered machine. He built the Blucher, the first successful flanged-wheel adhesion locomotive. The flanges enabled the trains to run on top of the rails instead of in sunken tracks. This greatly simplified construction of switches (called "points" in UK) and rails, and opened the way to the modern railroad.

Benefits of locomotives

The SL Yamaguchi-go near Tsuwano in West Japan

There are many reasons why the motive power for trains has been traditionally isolated in a locomotive, rather than in self-propelled vehicles. These include:

Classification by motive power

Locomotives may generate mechanical work from fuel, or they may take power from an outside source. It is common to classify locomotives by their means of providing motive work - the common ones include:

Steam

The main moving parts of a steam locomotive with Walschaerts valve gear: 1 - Link, 2 - Eccentric crank, 3 - Radius rod, 4 - Lap/lead lever, 5 - Crosshead, 6 - Valve, 7 - Cylinder, 8 - Reach rod

A steam locomotive at the Gare du Nord, Paris, France, in 1930

Hagley Hall steam locomotive on display in the restaurant area of the McArthur Glen Shopping Centre, Swindon, England

The first railway locomotives (19th century) were powered by steam, first by burning wood, later coke and coal or petroleum. Because of the steam engine, some people took to calling the steam locomotives themselves "steam engines". The steam locomotive remained by far the most common type of locomotive until after World War II. The age of steam correlates highly to the coal era.

The first steam locomotive was built by Richard Trevithick, and first ran on 21 February 1804, although it took some years before steam locomotive design became efficient and economically practical. Fairy Queen, built in 1855; plying between New Delhi and Alwar in India, is the longest-running steam locomotive in regular service in the world, but John Bull, built in 1831, is currently the oldest operable steam locomotive. John Bull is preserved in mostly static display at the Smithsonian Institution in Washington, DC.

The all-time speed record for steam trains is held by an LNER Class A4 4-6-2 Pacific locomotive of the LNER in the United Kingdom, number 4468 Mallard, which pulling six carriages (plus a dynamometer car) reached 126 mph (203 km/h) on a slight downhill gradient down Stoke Bank on 3 July 1938. Aerodynamic passenger locomotives from other countries such as Germany and the United States attained speeds very close to this, and this is generally believed to be close to the practicable upper limit for the direct-coupled steam locomotive.

Before the middle of the 20th century, electric and diesel-electric locomotives began replacing steam locomotives. Steam locomotives are less efficient than their more modern diesel and electric counterparts and require much greater manpower to operate and service. British Rail figures showed the cost of crewing and fuelling a steam locomotive was some two and a half times that of diesel power, and the daily mileage achievable was far lower. As labour costs rose, particularly after the second world war, non-steam technologies became much more cost-efficient. By the end of the 1960s-1970s, most western countries had completely replaced steam locomotives in commercial service. Freight locomotives generally were replaced later. Other designs, such as locomotives powered by gas turbines, have been experimented with, but have seen little use.

By the end of the 20th century, almost the only steam power still in regular use in North America and Western European countries was on heritage railways specifically aimed at tourists and/or railroad enthusiasts, known as railfans or train spotters, although some narrow gauge lines in Germany which form part of the public transport system, running to all-year-round timetables retain steam for all or part of their motive power. Steam locomotives remained in commercial use in parts of Mexico into the late 1970s. Steam locomotives are in regular use in China, where coal is a much more abundant resource than petroleum for diesel fuel. India has switched in the 1990's from steam-powered trains to electric- and diesel-powered trains. In some mountainous and high altitude rail lines, steam engines remain in use because they are less affected by reduced air pressure than diesel engines.

British Railways standard class 5 73096, a 4-6-0 steam loco, at Virginia Water station, April 2004.

See also

External links

Diesel-mechanical

A British Rail Class 03 diesel-mechanical switcher with a jackshaft under the cab.

Diesel locomotives vary in the form of transmission used to convey the power from a diesel engine (or engines) to the wheels. The simplest form of transmission is by means of a gearbox, in the same way as on road vehicles. Diesel trains or locomotives that use this are called diesel-mechanical and began to appear (although limited in power) even before the first world war which saw a number of simplex diesel systems built for the war, a small number of which survive and are still operational today. In the 1960s, the locomotive manufacturer Krauss-Maffei built a variant of this type of locomotive which used hydraulic power transmission for the Southern Pacific Railroad in the U.S., but the units were ultimately removed from service due to maintenance and reliability problems.

It has been found impractical to build a gearbox which can cope with a power output of more than 400 horsepower (300 kW) without breaking, despite a number of attempts to do so. Therefore this type of transmission is only suitable for low-powered shunting locomotives, or lightweight multiple units or railcars.

For more powerful locomotives, other types of transmission have to be used.

Diesel-electric

EMD SD70 series diesel-electric locomotives of the Union Pacific refueling at Dunsmuir, California

The most common form of transmission is electric; a locomotive using electric transmission is known as a diesel-electric locomotive. With this system, the diesel engine drives a generator or alternator; the electrical power produced then drives the wheels using electric motors. In effect, such a locomotive is an electric locomotive which carries its own generating station along with it.

Early diesel-electrics were switching engines used to move rail cars around in rail yards. The first went into service in 1918 with the Jay Street Connecting Railroad. Sixteen years later, the technology began to be applied to regular mainline service as streamlined passenger trains went into operation. Actually, a petroleum distillate-electric system powered the first such train, but diesel-electric systems soon proved to be more cost-effective because of higher efficiency and lower maintenance costs. The fuel for one early high-speed run from Chicago, Illinois to Denver, Colorado only cost US$14.64 (in 1934 dollars).

In the 1970s, British Rail in the United Kingdom developed a high-speed diesel-electric train called the High Speed Train or HST. This train consists of two Class 43 locomotives (also known as power cars), one at each end, and a number of "Mark 3" carriages (usually 8). A complete HST set was originally designated as a Class 253 or 254 diesel multiple unit (DMU), but due to the frequent exchanges between sets the power cars were reclassified as locomotives and given class number 43. The unpowered carriages were simultaneously reclassified as individual coaches - the number of a DMU set should identify all its associated carriages as well.

The prototype HST (designated Class 252) holds the world speed record for diesel traction, having reached a speed of 143 mph, although the operating speed of the production HST in service is 125 mph (200 km/h), hence the name "Inter-City 125".

A variant of the Intercity 125, the XPT, is in service on New South Wales railways in Australia, but with a lower top speed and different carriages.

Diesel-hydraulic

Alternatively, diesel-hydraulic locomotives use hydraulic transmission to convey the power from the diesel engine to the wheels. On this type of locomotive, the power is transmitted to the wheels by means of a device called a torque converter. A torque converter consists of three main parts, two of which rotate, and one which is fixed. All three main parts are sealed in a housing filled with oil.

The inner rotating part of a torque converter is called a centrifugal pump (or impeller), the outer part is called a turbine wheel (or driven wheel), and between them is a fixed guide wheel. All of these parts have specially shaped blades to control the flow of oil.

The centrifugal pump is connected directly to the diesel engine, and the turbine wheel is connected to an axle, which drives the wheels.

As the diesel engine rotates the centrifugal pump, oil is forced outwards at high pressure. The oil is forced through the blades of the fixed guide wheel and then through the blades of the turbine wheel, which causes it to rotate and thus turn the axle and the wheels. The oil is then pumped around the circuit again and again.

The disposition of the guide vanes allows the torque converter to act as a "gearbox" with continuously variable ratio. If the output shaft is loaded so as to reduce its rotational speed, the torque applied to the shaft increases, so the power transmitted by the torque converter remains more or less constant.

However, the range of variability is not sufficient to match engine speed to load speed over the entire speed range of a locomotive, so some additional method is required to give sufficient range. One method is to follow the torque converter with a mechanical gearbox which switches ratios automatically, similar to an automatic transmission on a car. Another method is to provide several torque converters each with a range of variability covering part of the total required; all the torque converters are mechanically connected all the time, and the appropriate one for the speed range required is selected by filling it with oil and draining the others. The filling and draining is carried out with the transmission under load, and results in very smooth range changes with no break in the transmitted power.

Diesel-hydraulic multiple units, a less arduous duty, often use a simplification of this system, with a torque converter for the lower speed ranges and a fluid coupling for the high speed range. A fluid coupling is similar to a torque converter but the ratio of input to output speed is fixed; loading the output shaft results not in torque multiplication and constant power throughput but in reduction of the input speed with consequent lower power throughput. (In car terms, the fluid coupling provides top gear and the torque converter provides all the lower gears.) The result is that the power available at the rail is reduced when operating in the lower speed part of the fluid coupling range, but the less arduous duty of a passenger multiple unit compared to a locomotive makes this an acceptable tradeoff for reduced mechanical complexity.

Diesel-hydraulic locomotives are slightly more efficient than diesel-electrics, but were found in many countries to be mechanically more complicated and more likely to break down. In Germany, however, diesel-hydraulic systems achieved extremely high reliability in operation. Persistent argument continues over the relative reliability of hydraulic engines, with continuing questions over whether data was manipulated politically to favour local suppliers over German ones. In the US and Canada, they are now greatly outnumbered by diesel-electric locomotives, while they remain dominant in some European countries. The most famous diesel-hydraulic locomotive is the German V200 which were built from 1953 in a total number of 136. The only diesel-electric locomotives of the Deutsche Bundesbahn were BR 288 (V 188), of which 12 were built in 1939 by the DRG.

The high reliability of the German locomotives was paralleled by higher reliability of non-German locomotives built with German-made parts compared to that of the same designs built using parts made locally to German patterns under licence. Much of the unreliability experienced outside Germany was due to poor quality control in the local manufacture of engines and transmissions, and poor maintenance due to staff used to steam locomotives working on unfamiliar and much more complex designs in unsuitable conditions and failing to follow the unit-replacement maintenance methods which were part of the German success. It is notable that diesel-hydraulic multiple units, with the advantages of modern manufacturing techniques and improved maintenance procedures, are now extremely successful in widespread use, achieving excellent reliability.

Gas turbine-electric

UP 68, one of Union Pacific's 4500hp 'veranda' turbines. From the Don Ross Collection

Main article: Gas turbine-electric locomotive

A gas turbine-electric locomotive, or GTEL, is a locomotive that uses a gas turbine to drive an electric generator or alternator. The electric current thus produced is used to power traction motors. This type of locomotive was first experimented with in 1920 but reached its peak in the 1950s to 1960s. The turbine (similar to a turboshaft engine) drives an output shaft, which drives the alternator via a system of gears. Aside from the unusual prime mover, a GTEL is very similar to a diesel-electric. In fact, the turbines built by GE used many of the same parts as their diesels.

A turbine offers some advantages over a piston engine. The number of moving parts is much smaller, and the power to weight ratio is much higher. A turbine of a given power output is also physically smaller than an equally powerful piston engine, allowing a locomotive to be very powerful without being inordinately large. However, a turbine's power output and efficiency both drop dramatically with rotational speed, unlike a piston engine, which has a comparatively flat power curve.

Gas turbine locomotives are very powerful, but also tend to be very loud. Union Pacific operated the largest fleet of such locomotives of any railroad in the world, and was the only railroad to use them for hauling freight. Most other GTELs have been built for small passenger trains, and only a few have seen any real success in that role. After the oil crisis in the 1970s and the subsequent rise in fuel costs, gas turbine locomotives became uneconomic to operate, and many were taken out of service. This type of locomotive is now rare.

Electric

Main article: Electric locomotive

Electric locomotive VL60k/p (60/)

The electric locomotive is supplied externally with electric power, either through an overhead pickup or through a third-rail. While the cost of electrifying track is rather high, electric trains and locomotives are significantly cheaper to run than diesel ones, and are capable of superior acceleration as well as regenerative braking, making them ideal for passenger service in densely populated areas. Almost all high speed train systems (e.g. ICE, TGV, Shinkansen) use electric power, because the power needed for such performance is not easily carried on board. For example the most powerful electric locomotives that are used today on the Channel Tunnel freight services use 7 MW of power.

The world speed record for a wheeled train was set in 1990 by a French TGV which reached a speed of 515.3 km/h (320 mph).

While recently designed electrified railway systems invariably operate on alternating current, many existing direct current systems are still in usee.g. in South Africa, Spain, and the United Kingdom (750 V and 1500 V); Netherlands (1500 V); Belgium, Italy, Poland (3000 V), and the cities of Mumbai and Chicago, Illinois (which will be switched to AC by 2025).

A small number of electric locomotives can also operate off battery power to enable short journeys or shunting to occur on non-electrified lines or yards. Pure battery locomotives also found usage in mines and other underground workings where diesel fumes or smoke are not safe and where external electricity supplies could not be used. Battery locomotives are also used on many underground railways for maintenance operations as they are required to operate in areas where the electricity supply has been temporarily disconnected.

See also: Railway electrification system

Electro-diesel

Main article: Electro-diesel locomotive

These are special locomotives that can either operate as an electric locomotive or a diesel locomotive. Dual-mode diesel-electric/third-rail locomotives are operated by the Long Island Rail Road and Metro-North Railroad between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in Manhattan tunnels. For the same reason Amtrak operates a fleet of dual-mode locomotives in the New York area. British Rail operated dual diesel-electric/electric locomotives designed to run primarily as electric locomotives. This allowed railway yards to remain un-electrified as the third-rail power system is extremely hazardous in a yard area.

Magnetic levitation

Transrapid maglev train on the test track at Emsland, Germany.

The newest technology in trains is magnetic levitation (maglev). These electrically powered trains have a special open motor which floats the train above the rail without the need for wheels. This greatly reduces friction. Very few systems are in service and the cost is very high. The experimental Japanese magnetic levitation train JR-Maglev MLX01 has reached 581 km/h (361 mi/h).

The transrapid maglev train connects Shanghai's airport with the city.

The first commercial maglev trains ran in the 1980s in Birmingham, United Kingdom, providing a low-speed shuttle service between the airport and its railway station. Despite the huge interest and excitement in the technology it was abandoned and replaced by a cable-hauled guideway a few years later.

Classification by use

The three main categories of locomotives are often subdivided in their usage in rail transport operations. There are passenger locomotives, freight locomotives and switcher (or shunting) locomotives. These categories mainly depend on manoeuvrability, traction power and speed. Some locomotives are designed to work in mountain railways.