RAILWAY ENGINEERING. IN: HANDBOOK OF TRANSPORTATION ENGINEERING

Abstract

This chapter describes how diesel-electric locomotives and electric locomotives are classified by wheel arrangement with letters representing the number of axles in a rigid truck (A for one axle, B for two axles, C for three axles, etc.). Idler axles between drivers may be numerals. A plus sign indicates articulated trucks or motive power units. A minus sign indicates separate nonarticulated trucks. This nomenclature is explained in RP-5523, issued by the Association of American Railroads (AAR). Virtually all modern locomotives are of either B-B or C-C configuration. The high efficiency of the diesel engine is an important factor in its selection as a prime mover for traction. This efficiency at full or partial load makes it ideally suited to the variable service requirements for routine railroad operations. The diesel engine is a constant-torque machine that cannot be started under load and hence requires a variably coupled transmission arrangement. The electric transmission system allows it to make better use of its full rated power output at low track speeds for starting as well as for efficient hauling of heavy trains at all speeds. Most diesel-electric locomotives have a dc generator or rectified alternator coupled directly to the diesel engine crankshaft. The generator/alternator is electrically connected to dc series traction motors having nose suspension mountings. Many recent locomotives utilize gate turn-off inverters and ac traction motors to obtain the benefits of increased adhesion and higher tractive effort. The gear ration for the axle-mounted bull gears to the motor pinions that they engage is determined by the locomotive speed range and this is related to the type of service. A high ratio is used for freight service where a high tractive effort and low speeds are common, whereas high-speed passenger locomotives have a lower ratio.