Locomotive is a machine that moves trains on railroad tracks. Early locomotives weighted from 3 to 6 short tons (2.7 to 5.4 metric tons) and could pull or push only a few light cars. A modern locomotive may weigh over 700 short tons (640 metric tons) and move more than 200 loaded freight cars at a time.
Locomotives designed to haul freight or passenger trains are called road locomotives. A yard switcher locomotive moves cars from track to track in railroad yards. A general purpose locomotive can be used to haul trains or for yard switching.
There are three main kinds of locomotives, depending on their source of power: (1) diesel, (2) electric, and (3) steam. A fourth kind, powered by machines called gas turbines, once hauled some freight in the United States. Railroads no longer use such locomotives, but gas turbines do power certain high-speed, lightweight passenger trains called turbotrains. Such turbines resemble those used in aircraft. Turbotrains do not have locomotives. Instead, their power units are built into one or more of the cars.
Steam locomotives once pulled most railroad trains, and they are still used in some countries. But in most countries, diesel locomotives have replaced other kinds almost entirely. In the late 1980's, railroads in the United States operated about 21,000 diesels and fewer than 100 electric locomotives. About 50 steam locomotives remained in operation. They were used chiefly to pull tourist trains.
Kinds of locomotives
Diesel locomotives are actually traveling power plants. They have a diesel engine that works by compressing air in chambers called cylinders. When air is compressed, its temperature rises. The resulting heat ignites fuel that has been injected into the cylinder. The power produced during this process is then transmitted to the locomotive's driving wheels.
Diesel locomotives have a number of advantages. They generate their own power and therefore can operate anywhere that there are rails. Diesel locomotives can also make long runs without refueling, or servicing. They can be quickly stopped or started, and they speed up faster than steam engines. They also have a higher fuel efficiency than steam locomotives, require less servicing, and cost less to maintain.
A diesel locomotive may be a combination of one to four or more connecting units. These units are of two general types, called A and B. An A unit is designed and equipped for use by itself or as a lead unit when a number of units are combined. A B unit does not have the engineer's cab and controls needed to serve as a lead unit, and it is capable of only limited independent movement. A diesel locomotive may consist of a single A unit or of two, three, four, or more A and B units coupled together.
In the United States, railroads often use locomotives with six or even more units to pull heavy trains at high speeds or on mountain grades. Sometimes one or more radio remote control units are also used on extremely long trains. These units are placed near the middle of the train. The engineer controls them by radio signals from his cab, which is located in the lead unit at the front of the train.
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Diesel locomotive units range from 400 to 6,600 horsepower (300 to 4,920 kilowatts) each. Most of those in freight and passenger train service run from 1,500 to more than 3,000 horsepower (1,120 to 2,200 kilowatts). Two 3,000-horsepower units coupled together make a 6,000-horsepower locomotive.
There are three types of diesel locomotives: (1) diesel-electric, (2) diesel-hydraulic, and (3) diesel-mechanical. Each transmits power from the engine to the driving wheels in a different way.
Diesel-electric locomotives are by far the most common type. Almost all locomotives in Ukraine are diesel-electrics. In these locomotives, the engine drives a machine called a generator, which produces an electric current. The current is then fed to traction motors, which drive gears that turn the locomotive's driving wheels.
Most diesel-electrics have generators that produce direct current (DC), a kind of current that flows in only one direction. Generators on some newer, large diesels produce alternating current (AC), which reverses direction many times every second. Most diesels have traction motors that operate on direct current. Therefore, locomotives with an AC generator must rectify (convert) the alternating current into direct current before it goes to the motors. Devices called silicon rectifiers perform this conversion.
Diesel-hydraulic locomotives. In this type of locomotive, the engine drives a torque converter instead of a generator. A torque converter is a device that uses fluids under hydraulic pressure to transmit and regulate power received from the engine. The converter includes a pump and a turbine. The turbine changes energy from the fluids into a force that can be used to perform work. The engine delivers oil to the converter and drives the pump. The pump forces the oil against the blades of the turbine. This action causes the turbine to rotate and to drive a system of gears and shafts that move the wheels.
Diesel-hydraulic locomotives are not used in the United States. However, they are widely used in some other countries, especially in Germany, where they first appeared.
Diesel-mechanical locomotives transmit power from the engine in much the same way that automobiles do-through a clutch and a system of gears and shafts. The clutch connects the engine to the transmission. The gears and shafts fit together to transmit the power and drive the wheels. This mechanical drive delivers less power than do other systems, and so it works well only on small locomotives.
Electric locomotives, unlike diesels, do not produce their own power. They use electric power supplied by a central power plant that may be miles away. Therefore, an electric locomotive needs special wires or rails from which it can get power.
Most electric locomotives in Ukraine use alternating current. They obtain the power from an overhead wire called a catenary. A hinged steel framework called a pantograph, which conducts electricity, connects this wire with the locomotive. Locomotives that operate on alternating current receive power at extremely high voltages. They have a device called a transformer that reduces the voltage to a level at which it can be used. The power is then fed into AC traction motors, or it is rectified and fed into DC traction motors.
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Some locomotives that operate on direct current also obtain power by way of a catenary and pantograph. Other DC locomotives use an electrical third rail, which runs parallel to the regular rails. A metal device called a contact shoe is attached to these locomotives. The shoe slides along the rail and picks up electricity from it.
An electrified rail network costs a tremendous sum to build because of the wires and other special equipment involved. However, electric locomotives can draw vast amounts of power from their central power plant. Diesels, on the other hand, are limited to the power they can produce from the fuel they carry. Electric locomotives can be started instantly. They also are quiet and produce no smoke or exhaust gases. Thus, they are sometimes used in heavily populated areas and on railroads that run underground or through long tunnels.
Like diesels, most electric locomotives can operate either singly or combined in groups of two or more units. However, electric locomotives can produce more power per unit than diesels can. Therefore, electric locomotives are especially useful for fast, heavy freight trains or high-speed passenger trains.
Many kinds of fuel can be used by a power plant to produce the electricity that runs an electric locomotive. For example, a power plant may use coal, gas, oil, water power, or atomic power. Diesel locomotives can run only on diesel oil, which someday may become scarce and costly. For this reason, some Ukrainian transportation experts believe that electrification of the nation's railroads may in time become widespread.
Steam locomotives produce heat by burning coal or fuel oil in a firebox. The heat turns the water in the locomotive's boiler into steam, which is fed into cylinders. There, the pressure produced by the steam drives steel rods called pistons. The pistons are connected to piston rods, main rods, and side rods, which move the driving wheels. A steam locomotive has an attached car called a tender that carries the fuel and water.
Steam locomotives have several disadvantages. For example, a steam locomotive needs frequent care, especially to keep the fire burning in the boiler. A long time is required to light the fire and to heat the boiler so that steam can be produced. In addition, steam locomotives cannot maintain the high average speeds of diesel or electric locomotives. They also have low fuel efficiency. A steam locomotive must burn large amounts of fuel to produce power, but little of the heat produced is used to run the locomotive. The rest is wasted.
