Locomotive

I

Introduction

Locomotive, any type of self-propelled vehicle used by railways to pull or push other types of rolling stock. The locomotive differs from other kinds of self-propelled railway vehicles in that it is used solely as a power unit and is not designed for carrying passengers or freight.

II

Early History

The first practical locomotive was constructed in England in 1804 by the engineer and inventor Richard Trevithick. This locomotive, with four driving wheels, had smooth wheels operating on smooth metal rails; its success proved that sufficient traction could be obtained without using gear wheels and a cogged or toothed track. Equally important, the Trevithick locomotive exhausted its steam into the smokestack of the engine's firebox; this provided a forced draught for the fire in the firebox and was employed on all subsequent steam locomotives.

After the successful trials of the Trevithick locomotive, a number of moderately successful locomotives were built in Britain, primarily for use in mining. Not until 1829 was a locomotive developed for use in a railway carrying both passengers and freight. In that year the Rocket, designed by George Stephenson, won a competition sponsored by the Liverpool and Manchester Railway. The Rocket pulled a load of three times its own weight at 20 km/h (12.5 mph) and hauled a coach filled with passengers at 39 km/h (24 mph). This performance stimulated the building of other locomotives and the extension of railway lines.

Also in 1829, the first locomotive to operate in the western hemisphere was given a trial at Honesdale, Pennsylvania. This locomotive, the Stourbridge Lion, had been built in England for the Delaware and Hudson Canal Company. In the following year, the first locomotives built in the United States were put into operation: The Best Friend was operated by the South Carolina Canal and Railroad Company, and the Peter Cooper, also known as the Tom Thumb, was operated by the Baltimore and Ohio Railroad Company. The Peter Cooper was little more than an enlarged model, but it outperformed the Rocket. Old Ironsides, built by the American industrialist Matthias William Baldwin for the Philadelphia, Germantown & Norristown Railroad Company, was a four-wheeled locomotive weighing about 5 tonnes. It was given its first track tests in 1832 and put into service almost immediately.

Many mechanical improvements were subsequently made, both in Britain and in the United States. These two countries have generally paralleled each other in locomotive development. In 1831 the swivelling undercarriage,or bogie, supplanted the fixed bogie; in 1836 the outside coupling of pairs of driving wheels was introduced; and in 1837 counterbalances were applied to driving wheels and other parts to smooth the operation of the engine. The first locomotive with six driving wheels and a four-wheeled leading bogie, often called a ten-wheeler, appeared in 1847. In 1863 locomotive types with six driving wheels and a two-wheel leading bogie came into use; and in 1867 the first locomotive with eight coupled drivers and a two-wheeled leading bogie was built.

III

Steam Locomotives

Steam locomotives can be classified in a number of ways. The most generally used classification, however, is based on the number and arrangement of wheels. This classification gives the number of wheels on the leading bogie, the number of driving wheels, and the number of wheels on the trailing bogie. Thus, a 2-4-0 locomotive is one with a two-wheel leading bogie, four driving wheels, and no trailing bogie. Many locomotives are also given special type names.

Until about 1940, steam engines provided the driving power of most locomotives used on railways. Subsequently, the steam locomotive became largely obsolete, first in the United States and then elsewhere. By the late 1980s, only a few, such as those running on narrow-gauge tourist lines, were operating in the industrialized countries.

IV

Diesel-Electric Locomotives

Among important locomotives developed in the 20th century are the electric locomotive, which picks up electrical power from an overhead wire or a third rail laid beside the track (see Railway), and the diesel-electric locomotive. In diesel-electric locomotives, known popularly as diesels, diesel engines are used to drive generators or alternators connected to solid-state rectifiers that power electric motors geared to the axles (see Internal-Combustion Engine). This type of locomotive eliminates the need for expensive power-transmission lines. Compared to the steam locomotive, it has a greater availability, meaning a higher average of productive hours per day, because it does not require frequent stops for water, fuel, and other servicing. Other advantages over the steam engine include its relative efficiency in converting fuel into available energy and its ability to develop a much higher proportion of its maximum pulling power at low speeds. Also, while steam locomotives require a driver and fireman for each locomotive, diesel-electrics can be operated as multiple units by one individual, resulting in longer trains and fewer employees.

Recent locomotive designs make use of improved turbochargers coupled with more powerful and more fuel-efficient engines. Locomotive control systems have been converted to solid-state electronics, replacing most relay functions. On-board microprocessors control engine speed, fuel injection, and excitation of the alternator. These computers also interconnect with improved systems to detect slipping or sliding of the driving wheels, producing faster correction and improved adhesion. An additional function of the microprocessor is to monitor performance of all locomotive systems, thereby increasing their reliability and making the correction of problems easier. An important innovation in the locomotive is the introduction of variable-frequency, variable-voltage, three-phase alternating-current traction motors in place of direct-current series motors, reducing weight and improving wheel adhesion.