Diesel locomotive
Diesel locomotives have become the dominant type of
locomotive in
rail transport. Powered by
diesel engines, they use a variety of
transmissions to convey power to the wheels.
Rudolf Diesel himself suggested that his engines could be put to railroad use, and in 1909 helped construct an experimental locomotive.
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 (although limited in power) even before
World War I which saw a number of simplex diesel systems built for the war, a small number of which survive and are still operational today.
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 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
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.
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 the
pioneer Zephyr high-speed run from
Chicago, Illinois to
Denver, Colorado only cost
US$14.64 (in 1934 dollars).
The
General Motors Electro-Motive Division (now
Electro-Motive Diesel, Incorporated) arose to become the world's largest builder of diesel engines. Their E and F series and SD40s and 60's are famous (see
List of GM-EMD locomotives. Other makers included
General Electric,
Fairbanks-Morse,
ALCO and the
Canadian Locomotive Company.
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 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".
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.
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, 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.
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.
When mainline diesels were mass produced in the United States, they were sold as multiple unit sets. The engines and traction motors of the day were not capable of the kind of 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 so-called "
American Association of Railroads" standard for multiple-unit control reamins the basis for US operation. The Kraus-Maffei diesel-hydraulic units were also equipped with this system. See also
Multiple working for UK locomotives.
