Locomotive





A locomotive is a vehicle that provides the motive power for a railway
train. Traditionally, the locomotive or locomotives are positioned at the
front of a train, pulling passenger carriages and/or freight vehicles. This
requires the locomotive to be moved from one end of the train to another
when a change of direction is required. However, it is now common for the
locomotive for a passenger train to remain at the same end of the train. A
driving cab is provided in the outermost carriage, with controls which
communicate with the locomotive through wiring along the train. The train is
thus pulled by the locomotive when moving in one direction, and pushed in
the other. A variation of this occurs when a train consists of a set of
carriages with a locomotive at each end, both of which are controlled by a
driver in the leading locomotive.

Drivers are instructed by signals when they may start or must stop. see
railway signaling

Long freight trains sometimes have locomotives at the front and rear, and
even in the middle of the train. This reduces the force on the couplings
between the freight vehicles. In this case, control signals are usually sent
from the leading locomotive by radio.

Trains which do not have locomotives are referred to as multiple units.

Steam

The first railway locomotives (19th century) were powered by steam engines.
Because of this, the some people took to informally 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 all-time speed record for steam trains is held by an A4 Pacific class
locomotive of the LNER in England, Mallard which pulling six cars (plus a
dynamometer car) reached 203 km/h (126 mph) on a slight downhill gradient
down Stoke Bank on July 3, 1938. Locomotives from other nations 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 locomotives began
replacing steam locomotives. By the end of the 1960s, most countries had
completely replaced steam locomotives in commercial service. Other designs,
such as locomotives powered by gas turbines, have been experimented with,
but seen little use.

Well before the end of the 20th century, almost the only steam power still
in regular use in the USA and Western European countries was on railroads
specifically aimed at tourists and/or railfans. Steam locomotives remained
in commercial use in parts of Mexico in to the late 1970s. Steam locomotives
remain in regular use in China, where coal is a much more abundant resource
than petroleum for diesel fuel. In some mountainous and high altitude rail
lines, steam remains in use because it is less affected by reduced air
pressure than is diesel.

Diesel-mechanical

Diesel locomotives differ in the form of transmission used to convey the
power from a diesel engine (or engines) to the wheels. The most simple form
of transmission is by means of a gearbox, in the same way as on road
vehicles. Diesel trains or locomotives which use this are called
diesel-mechanical.

It has however, been found impossible to build a gearbox which can cope with
a power output of more than 400 horsepower 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; 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.

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 is fixed, all of which are sealed in a housing
filled with oil.

The inner rotating part of a torque converter is called a centrifugal pump
(or impellar), the outer part is called a turbine wheel (or driven wheel),
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.

Diesel-hydraulic locomotives are slightly more efficient than
diesel-electrics but are mechanically more complicated and more likely to
break down. They are now greatly outnumbered by diesel-electrics.

Gas Turbine

Locomotives powered by gas turbines, were developed in many countries in the
decades after World War II. These used jet-type engines (similar to the
turboshaft engines in a turbine helicopter) driving an output shaft. The
normal method of transmitting power to the wheels involved an electrical
transmission similar to a diesel-electric locomotive -- the turbines running
at constant speed driving a generator, feeding to large electric motors
driving the wheels.

Gas turbine locomotives are very powerful, but also very noisy (they sounded
rather like a jet aircraft). Their efficiency was quite low, but this was
initially not a problem; fuel was cheap, and some gas turbines were fuelled
with cheap 'Bunker C' heavy oil. This cheap fuel source vanished when
improved refinery techniques allowed it to be 'cracked' into lighter
petroleum grades. After the oil crisis in the 1970s and the rise in fuel
costs, gas turbine locomotives became uneconomic to run, and many were taken
out of service. This type of locomotive is now rare.

Electric

Electric locomotives are externally supplied 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, bullet train) use electric locomotives, because the
power needed for such performance is not easily carried on board.

However in the 1970s British Rail in the United Kingdom, developed a
high-speed diesel electric locomotive called the High Speed Train, which is
capable of reaching speeds of up to 284 km/h (176 mph), although in service
it only reaches speeds of 200km/h (125 mph).

The world speed record for a wheeled train was set in 1990 by a French TGV
which reached a speed of 515 km/h (320 mph).

While recently designed electrified railway systems invariably operate on
alternating current, many existing direct current systems are still in
use?e.g. in South Africa, Spain, Switzerland and the United Kingdom,
Netherlands (1500 V), Belgium (3000 V).

Magnetic Levitation

The newest technology in locomotives 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 has reached 552 km/hr.