ENGINE

Steam Engine—The e ential principle in all types of steam engine is that they convert the energy stored up in steam into mechanical work. As early as 120 B.C., Hero of Alexandria employed steam to drive a kind of steam turbine; but practically no further progre was made for upwards of seventeen centuries. In 1629 Branca, an Italian, caused a wheel to rotate by means of a jet of steam impinging on vanes set on its rim. This was merely a scientific toy. Denis Papin 1690 invented the steam cylinder and piston, raising the piston by steam and then condensing the steam to form a vacuum. The first practical steam engine was made by Savery, for pumping purposes, in1698. It consisted of two egg-shaped ve els which could be connected alternately with a boiler and a well. Steam was admitted to one of the ve els, and, communication with the boiler having been shut off, was condensed there by the cooling action of a jet of water on the outside. A partial vacuum was thus formed, and water from the well was forced up the pipe by atmospheric pre ure. The ve els-acted alternately, one emptying while the other filled. The first practical piston engine was designed by Newcomen, Savery and Cawley 1705. The piston was connected with one end of an overhead rocking beam, the other end of which carried a long pump rod and a heavy weight or counterpoise to bring the piston to the top of its stroke when steam was admitted into the cylinder. That point of the stroke having been reached, the steam was shut off and condensed in the cylinder by a jet of cold water so as to form a partial vacuum under the piston, which was then forced down by atmospheric pre ure. The epoch-making inventions, patented between 1763 and 1769, by James Watt of Glasgow, revolutionized the steam engine, and are the basis of the perfect machines of to-day. Watt’s improvements were based on the following important principles; first, that the cylinder should be kept as warm as po ible to prevent undue lo of steam; second, that condensation should therefore take place in a ve el separate from the cylinder; third, that the steam should be used expansively to pre on the piston in the cylinder; and fourth, that the piston.and other parts should be made air and water tight, for which purpose he employed oil, animal fats, ete. The high-pre ure engine is due to Trevithick, whose ‘Cornish’ engine was provided with a single cylinder and condenser and used the steam expansively. The principle of the compound engine, first patented by Horn blower 1782, is a most important one, and its adoption marked a signal advance. It depends upon the fact that the economical working of an engine is greatly augmented by using steam at a high initial pre ure and allowing it to expand gradually to a low final pre ure. If all the expansion take place in one cylinder, there is a wide variation of temp. internally, and part of the entering steam is condensed without doing any work. By dividing the expansion into two or more stages carried out in two or more cylinders, the compound engine reduces the waste of steam due to condensation in the cylinder. Practically all modern engines are designed on the compound system, with the initial and the final expansions taking place in the ‘high pre ure’ and the ‘low-pre ure’ cylinders respectively. Triple-expansion engines have an intermediate cylinder between these two. Horn blower’s first compound engine was not commercially succe ful owing to patent difficulties. But the principle was revived later for marine purposes. In 1802 Symington, a Scot, engineer, ran the first steamer, the Charlotte Dundas, on the Forth and Clyde Canal; the Clermont was produced in 1807 by Fulton (See Rosert Furron), an American, and engined by Boulton and Watt; and the Comet, built on the Clyde! by Henry Bell in 1812, was so succe ful as practically to solve the problem of the application of steam to the propulsion of ships, while in 1829 the achievements of Stephen son’s Rocket ensured the adoption of locomotive traction on Jand. Thermodynamics of the Steam Engine. —In the steam engine, as in nearly every other type of heat engine, the expansion of an elastic fluid is utilized to do work, generally by acting on a piston which moves in a cylinder. The e entials in this action are three in number—viz., a working fluid, a source of heat, and a receptacle for unused heat. The working fluid is the steam, the source of heat the furnace, and the receptacle for unused heat the condenser. Heat is supplied to the cold water in the boiler; the water is converted into steam, pa es into the cylinder, expands, pushing the piston before it and becoming cooler, and is expelled into the condenser to be condensed into water again. Of the heat originally imparted to the steam, part is lost by conduction through the walls of the cylinder, part is carried into the condenser, and part disappears. This last portion is the part of the heat which has been employed to do work on the piston, and the work done in moving the piston through its outward stroke is the exact mechanical equivalent of the heat lost. The ratio of the heat so utilized to the total quantity of heat originally supplied to the steam is the measure of the thermal efficiency of the engine; in other words, the more heat used to drive the piston and the le heat conducted through the walls of the cylinder and expelled to the condenser the more efficient is the engine. In practice there is always a certain amount of heat unavoidably lost owing to the friction of the mechanical parts. The most important cause of wasteful lo of heat, however, is condensation in the cylinder; and this is now largely reduced by using superheated steam at a high temp. and pre ure. Important Engineering Definitions.— Energy is capacity for doing work; work fis done when a force acts through a distance overcoming resistance. The unit of work is the foot-pound—+.e., the amount of work done in raising a ma of 1 lb. to a height of 1 foot against gravity; or more generally, the amount of energy expended when a force of 1 lb. acts through a distance of 1 foot. The power or activity of an agent is its rate of doing work. The unit of power is the horsepower: an engine has 1 h.p. when its rate of working is 550 ft.-lb. per second, or 33,000 ft.-lb. per minute. Heat, like electricity, is a form of energy; and the unit of heat is the quantity required to raise the temp. of 1 lb. of water through 1° F. Heat energy may be transformed into mechanical energy, and conversely. It is found that for every unit of heat, so transformed 774 ft.-lb. of mechanical energy are produced; this quantity, first ascertained experimentally by Joule, is known as the mechanical equivalent of heat. The power of an engine is sometimes expre ed as indicated h.p., some times as brake h.p. The i.h.p. is the full power which the engine should develop in view of the pre ure on the piston. But a considerable portion oft this power, amounting to from 5 to 20 r cent. of the whole, is not available ‘or external work, being used up internally in overcoming the frictional resistances of the mechanism. The power the engine can develop for external use is called the brake h.p., and is determined by fitting a brake on the fly-wheel and measuring the energy absorbed by it. The ratio of b.h.p.to i.h.p.in any particular case is the measure of the mechanical efficiency of the engine. Stationary Engines are used to supply power to machines in factories power stations, workshops, etc. They are almost always of the direct-acting type, and may be simple, compound, or triple expansion according to the work they have to do. See Tursings, also Locomorrves and MarineE ENGINES,