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steam engine,machine for converting heat energy into mechanical energy using steam as a medium, or working fluid. When water is converted into steam it expands, its volume increasing about 1,600 times. The force produced by the conversion is the basis of all steam engines. Steam engines operate by having superheated steam force a piston to reciprocate, or move back and forth, in a cylinder. The piston is attached by a connecting rod to a crankshaft that converts the back-and-forth motion of the piston to rotary motion for driving machinery. A flywheel attached to the crankshaft makes the rotary motion smooth and steady. The typical steam engine has an inlet valve at each end of the cylinder. Steam is admitted through one inlet valve, forcing the piston to move to the other end of the cylinder. This steam then exits through an exhaust valve. Steam from the other inlet valve then pushes the piston back to its original position, and the cycle starts again. In a single-cylinder steam engine the exhaust steam is usually expelled directly into the atmosphere. A compounded steam engine has several cylinders, which the steam passes through successively until, leaving the last cylinder, it is condensed into water and returned to the boiler. From the Greek inventor Heron of Alexandria to the Englishmen Thomas Newcomen and John Cawley, many persons contributed to the work of harnessing steam. However, James Watt's steam engine, patented in 1769, provided the first practical solution. Earlier engines depended on atmospheric pressure to push the piston into the cylinder, where a vacuum was created by sudden cooling of its steam content. Watt's use of a separate condenser resulted in a 75% saving in fuel. It also made possible the use of steam pressure to move the piston in both directions. Watt's continuing efforts produced a governor, a mercury steam gauge, and a crank-flywheel mechanism, all of which prepared the steam engine for a major role in the Industrial Revolution. Sailing vessels gave way to steamboats, and stagecoaches yielded to railroad trains as the steam engine was perfected. Transmitted by belts, ropes, shafts, pulleys, and gears, the energy from steam engines drove machines in factories and mills. Now, however, steam engines have been replaced in most applications by more economical and efficient devices, e.g., the steam turbineturbine,
rotary engine that uses a continuous stream of fluid (gas or liquid) to turn a shaft that can drive machinery.
A water, or hydraulic, turbine is used to drive electric generators in hydroelectric power stations.
..... Click the link for more information. , the electric motor, and the internal-combustion engine, including the diesel engine. They are still sufficiently economical to be used in industries where steam is necessary for some purpose in addition to that of driving an engine.
See C. W. Pursell, Early Stationary Steam Engines in America (1969); E. Robinson, James Watt and the Steam Revolution (1969); see also bibliography under locomotivelocomotive,
vehicle used to pull a train of unpowered railroad cars. Types of Locomotives
The steam-powered locomotive played a key role during the development and golden age of railroading, but, despite its long and picturesque history, it has been superseded in
..... Click the link for more information. .
a prime piston engine for converting the potential heat energy (pressure) in steam to mechanical energy. The operation of a steam engine is based on periodic changes in vapor pressure within the cylinder. The volume of the cylinder varies as the piston moves back and forth. Steam enters the cylinder of the steam engine, expands, and displaces the piston; a crank gear transforms the reciprocating motion of the piston into the rotary motion of the shaft (see Figure 1). The admission and release of steam is accomplished by the steam-distribution system. The cylinders of a steam engine have steam jackets to reduce thermal losses.
The first steam engine to have practical value as a general-purpose power source was designed by J. Watt between 1774 and 1784. Pioneers of the steam engine included D. Papin, who invented the steam boiler in 1680; T. Newcomen, who invented the atmospheric steam engine for raising water in 1705; I. I. Polzunov, who designed a steam blower between 1763 and 1765. The steam engine was the first—and, until the late 19th century, practically the only—general-purpose engine and played a singular role in the progress of industry and transportation.
In time, stationary steam engines were developed for use in factories and electric power plants. Locomotive steam engines were developed for railroad use, and marine steam engines were designed for merchant ships and military vessels. Mobile steam power units came to be used in agriculture and local industry.
Steam engines achieved a high degree of reliability and sophistication as early as the second half of the 19th century. Beginning with the 20th century, however, they met with increasing competition from rapidly evolving steam turbines and internal-combustion engines. The drawbacks of steam engines include low efficiency (from 1 to 20 percent), limited speed (up to 1,000 rpm), and total power output (up to 30,000 hp), size, and weight. As a result, the production of steam engines ended in the mid-20th century. Steam engines are found today (1975) only in steam locomotives, in steam power units, and on old steamships.
REFERENCESGrinevetskii, V. I. Parovye mashiny, 2nd ed. Moscow, 1926.
Obshchaia teplotekhnika, 2nd ed. Moscow-Leningrad, 1952.
Zhiritskii, G. S. Parovye machiny, 6th ed. Moscow-Leningrad, 1951.
S. M. LOSEV
steam engine[′stēm ¦en·jən]
A machine for converting the heat energy in steam to mechanical energy of a moving mechanism, for example, a shaft. The steam engine can utilize any source of heat in the form of steam from a boiler. Most modern machine elements had their origin in the steam engine: cylinders, pistons, piston rings, valves and valve gear crossheads, wrist pins, connecting rods, crankshafts, governors, and reversing gears. See Boiler, Steam
The 20th century saw the practical end of the steam engine. The steam turbine replaced the steam engine as the major prime mover for electric generating stations. The internal combustion engine, especially the high-speed automotive types which burn volatile (gasoline) or nonvolatile (diesel) liquid fuel, has completely displaced the steam locomotive with the diesel locomotive and marine steam engines with the motorship and motorboat. Because of the steam engine's weight and speed limitations, it was also excluded from the aviation field. See Diesel engine, Gas turbine, Internal combustion engine, Steam turbine.
A typical steam reciprocating engine consists of a cylinder fitted with a piston (Fig. 1). A connecting rod and crankshaft convert the piston's to-and-fro motion into rotary motion. A flywheel tends to maintain a constant-output angular velocity in the presence of the cyclically changing steam pressure on the piston face. A D slide valve admits high-pressure steam to the cylinder and allows the spent steam to escape (Fig. 2). The power developed by the engine depends upon the pressure and quantity of steam admitted per unit time to the cylinder.
Engines are classified as single- or double-acting, and as horizontal (Fig. 1) or vertical depending on the direction of piston motion. If the steam does not fully expand in one cylinder, it can be exhausted into a second, larger cylinder to expand further and give up a greater part of its initial energy. Thus, an engine can be compounded for double or triple expansion.
Steam engines can also be classed by functions, and are built to optimize the characteristics most desired in each application. Stationary engines drive electric generators, in which constant speed is important, or pumps and compressors, in which constant torque is important.