Template:Globalize/USA Template:Globalize/UK Diesel locomotives became the dominant type of locomotive in rail transport in the mid 20th century in much of the world. Powered by diesel engines, they use a variety of transmissions to convey power to the wheels. Diesel locomotives, in contrast to electric locomotives, do not require catenary installations to run. Therefore, they offer more flexibility in various types of service and are generally predominant in countries which, for historical or economical reasons, have few electrified lines. Since the 1950s, however, they have been superseded by electric locomotives in terms of power, maximum speed, tractive effort, and acceleration.
History
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Rudolf Diesel had suggested that his engine could be employed in railroad service, and in 1909 helped to construct an experimental locomotive.
In 1918 diesel electric switching locomotives were put in service in the United States. Sixteen years later, mainline engines began to be produced, at first for passenger service. Custom units were produced at at first for the Chicago, Burlington and Quincy Railroad's Pioneer Zephyr and as a single unit for the Baltimore and Ohio Railroad. Mass production of passenger and freight units soon followed. By 1960, diesel-electrics had displaced steam locomotives on every Class I railroad in the United States of America.
United Kingdom
In the 1970s British Rail 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 HST holds the world speed record for diesel traction, having reached a speed of 148 mph, although the operating speed in service is 125 mph (200 km/h), hence the name "Inter-City 125".
Transmission types
Unlike steam engines, diesel engines require a transmission to power the wheels. The engine must continue to idle when the locomotive is stopped.
Diesel-mechanical
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 after World War I.
It has been found impractical to inexpensively build a gearbox which can cope with a power output of more than 400 horsepower (300 kW) without failure, 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
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. This is effectively an electric locomotive with its own generating station.
For the first decades the motors were direct current. More recently alternating current has come to be preferred. In either case, a common option is the use of dynamic braking, in which the motors are switched to perform as generators, thus converting the motion of the locomotive into electrical energy, which is then dissipated through heating elements usually mounted on the top of the locomotive. Dynamic braking reduces brake usage in mountainous areas, though it is ineffective at low speeds.
Electro-diesel
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.
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 systems, 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. Another contributing factor was poor maintenance due to staff accustomed to steam locomotives now 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.
In the 1960s, more than 15 diesel-hydraulic locomotives were purchased by the Denver & Rio Grande and Southern Pacific Railroads on a trial basis from the Kraus-Maffei company. Only the outer shell of one of these (converted into a camera car by SP in the 1970s) exists today, the others having all been scrapped.
Multiple unit operation
When mainline diesels were mass produced in the United States, they were initially sold as multiple unit sets. The engines and traction motors of the day were not capable of the power output needed to pull an entire train with a single unit. These units were controlled through the same type of multiple unit system already in use for electric locomotives. The "American Association of Railroads" standard for multiple-unit control remains the basis for US operation. The Kraus-Maffei diesel-hydraulic units were also equipped with this system. See also Multiple working for UK locomotives.