This diesel-electric locomotive hauls a passenger train. Diesel engines spin generators that produce electricity to power the locomotive. They are more efficient than steam locomotives and do not require the overhead power lines needed for electric engines.
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-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. 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 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 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. 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.