A steam engine is a prime piston engine for converting the potential heat energy of pressurized steam to mechanical energy. Steam is water in gaseous state and, as all gases, it tends to expands in every direction; the higher the temperature, the greater the pressure the gas will exert on all sides of the vessel in which it is contained. This expansive pressure of steam is harnessed to push a piston and set it in motion to transmit movement.
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 is produced by a boiler. Then it 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. 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 James Watt between 1774 and 1784. Pioneers of the steam engine included Denis Papin, who invented the steam boiler in 1680; T. Newcomen, who invented the atmospheric steam engine for raising water in 1705. 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. It boosted the steel industry for the construction of railway tracks and bridges and steam ships. 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.
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| Steam engine parts: 1) piston; 2) connecting rod; 3) crankshaft; 4) flywheel. |