LOCOMOTIVE

I INTRODUCTION

Locomotive, any type of self-propelled vehicle used by railroads to pull or push other types of rolling stock, including passenger, freight, and work cars. The locomotive differs from other kinds of self-propelled railroad vehicles in that it is used solely as a power unit and is not designed for carrying passengers or freight, as are trolley cars and some electric cars.

II 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 with which the engine is equipped. Thisclassification 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. Switch engines, used in railroad yards, are usually of the 0-6-0 or 0-8-0 type. Passenger locomotivesinclude the American, 4-4-0; the Northern, 4-8-4; the Atlantic, 4-4-2; the Pacific, 4-6-2; and the Mountain 4-8-2. Freight locomotives include the Mogul, 2-6-0; the Consolidation, 2-8-0; the Decapod, 2-10-0; the Mikado, 2-8-2; and the Santa Fe, 2-10-2. A special type of locomotive used for heavy freight hauling is the articulated, or Mallet, locomotive, which is made up of two or more separate engines joined together, each with its own set of driving wheels. Among the various Mallet types are 0-6-6-0, 0-8-8-0, 2-6-6-2, 2-8-8-2, 2-10-10-2, and 2-8-8-8-2. Until about 1940, steam engines provided the driving power of most locomotives used on U.S. railroads. Subsequently, the steam locomotive became largely obsolete. By the late 1980s, only a few of them, such as those on the narrow-gage tourist routes of Colorado, were operating in the U.S.
III 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, and the diesel-electriclocomotive. 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. 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 hoursper 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 an engineman and fireman for each locomotive, diesel-electrics can be operated as multiple units by one individual, resulting in longer trains and fewer employees. See also Internal-Combustion Engine.

Diesel-electric locomotives were brought into service in the U.S. in 1925 for yard switching. The first passenger-train diesel unit was put into operation in 1934, and the first unit specifically designed for freight service came into use in 1941. Before World War II, the number of diesel units in service totaled about 800, as against more than 40,000 steam locomotives. In the late 1970s more than 27,000 diesel units were in operation, accounting for almost all U.S. railway motive power. By 1990, advances in technology allowed three new-generation locomotives to do the work of four older ones. Although railroads in 2000 hauled a record 1.5 trillion ton-miles, the locomotive fleet stood at only 20,028. 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 systemsto 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. A recent 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 unsprung weight and improving wheel adhesion
IV TURBINE-ELECTRIC LOCOMOTIVES
After World War II, extensive research in combustion engineering aided the development of turbine-electric locomotives, in which either gas or steam turbines were used to drive generators powering electric motors. A boiler was required to produce the steam in a steam turbine. In the gas turbine, gas was produced in a combustion chamber directly ahead of the turbine wheel. Either coal or oil may be used as fuel to produce steam or gas for turbine operation. Liquid propane gas has been used experimentally as fuel for gas turbines. All of these types of turbine-powered locomotives have been judged to be uneconomical for general freight rail operations. Only the gas turbine, driving wheelsthrough hydraulic transmission, has continued in service in the 1990s as the motive power in the form of a power car hauling a lightweight passenger train.