Page:The New International Encyclopædia 1st ed. v. 18.djvu/609

STEAM ENGINE. 538 STEAM ENGINE. ton is )0 X y. If now the total travel or stroke of the piston is .s feet, then the foot poumls of work for such stroke is represented by lo X y X s, and if the piston makes n strokes per minute, the foot pounds of work of the piston per min- xite are represented by u- X y X s X n. This amount, divided by 33,000, gives the horse power of the engine. Assuming all of the other factors to remain the same or to be constant, the horse power of an engine may be varied by vary- ing either the area of the piston, the length of the stroke, the pressure of the steam, or the number of strokes per minute. The whole of the theoretical horse power, however, is never avail- able for useful work, owing to the friction of the moving parts and other causes. Classification. Steam engines may be classi- fied (1.) according to the nature of their mecha- nism, (II.) according to the manner in which the steam is used, and (III.) according to the use to which the engine is put. I. Reciprocating engines may be grouped ac- cording to the position of the axis of the piston into horizontal engines, vertical engines, and in- clined or diagonal engines. Fig. 2 shows the posi- tion of the parts of a horizontal engine, that is, an engine with the axis of the cylinder and piston horizontal. The advantages of such an engine are convenient access from the grjaund to all parts of the mechanism, distribution of the weight over a large area, and the location of the centre of gravity of the machine close to the foimda- tion; its disadvantages are greater friction and wear of the cylinder owing to the fact that the piston is supported upon the bottom of the cyl- inder. In the vertical engine the axis of the cylinder and piston is vertical ; if the cylinder is carried by a frame above the crank shaft so that the pis- ton rod extends downward, the engine is called an iiirerted vertical oigine; and if the cylinder is below, with the piston rod extending upward to a crank shaft above, the engine is called a direct vertical engine. The advantages of the vertical engine are that the cylinder friction and consequent cylinder wear of the horizontal en- gine are largely avoided and that a comparatively small area of ground is occupied by the engine. Its disadvantages are that the weight of the piston, piston rod, crosshead, and connecting rod acts with the steam in one direction and against the steam in the other direction, thus producing unequal effort on the crank on alternate strokes, which has to be counteracted by special construc- tions; that the different parts of the mechanism are at different heights, requiring more attend- ants ; that the centre of gravity of the machine is high, reducing its stability. The limited foun- dation area required for the vertical engine espe- cially adapts it to places where room is scarce or ex])ensive, as in steamships and city power houses. See STE.iM Navigation. Inclined engines possess in a measure the ad- vantages and disadvantages of both the horizontal and the vertical engine. In such engines the axis of the cylinder is at an inclination between the vertical and horizontal, and the cylinder is usu- ally set below with the piston rod projecting diag- onally upward to the crank shaft. They are used chiefly in side-wheel .steamboats of shallow draught and in ferry-boats. An engine in which the motion of the piston Vol. XVIII— 34. is transmitted by a connecting rod direct to the crank, as shown by Fig. 2, is called a direct- acting engine. As applied to pumps this terra has a somewhat different meaning. (See Pumps ANn Pumping Machinery.) In contradistinction to direct-acting engines we liave beam engines, of which the engine shown by Fig. 1 is an early example. In the beam engine the coiniecting rod is journaled to one end of a beam rocking on a horizontal axis. To the other end of the beam is journaled a- pitman rod which connects with the crank slhTft. The engines for side-wheel steam- boats are usually beam engines; and they are al.so used for jjumping engines. Besides the forms of engines mentioned there are various special forms, a few of which are sufficiently important to merit mention. The oscillating engine dispenses with the connecting rod, the piston rod connecting directly to the crank, and the cylinder is mounted upon suitable trunnions so that it can oscillate in conformation lo the swaying of the piston rod in operating the crank. The trunk engine dispenses with the pis- ton rod, the connecting rod being attached direct to the piston, which takes the form of a hollow cylinder closed at one end. The back-acting engine has the connecting rod extending back from the crosshead to a crank shaft back of the engine instead of extending forward as show'n by Fig. 2. II. The method in which the steam is used in the engine is also a basis for a scheme of classifi- cation. The fact that the same number of foot pounds of energy per minute may l^e secured by a small piston working at high speed or a larger piston working at a slower speed gives us our first classification, viz., high-speed and loio-speed en- gines. The high-speed engine has the advantage of small dimensions and small weight for a given power, and, because the strolies are so frequent, of meeting variations in resistance more quickly than a slow-speed engine. Its disadvantages are its comparatively greater waste of steam, the greater wear of the moving parts, the greater danger of heating, and consequently the higher cost of construction and operation. Altogether, experience shows the slow or moderate speed en- gine to be superior to the high-speed engine where circumstances permit its use. Between 600 and 800 feet per minute is regarded as a moderate piston speed, and over 900 feet per minute as a high piston speed. Referring to Fig. 2, it y be observed that the steam acts first against one side of the piston and then against the other side. Such an engine is called a double-acting engine. When the pressure of the steam is exerted against one side of the piston only the engine is called a single-acting engine. The single-acting prin- ciple is utilized in the well-known Cornish engine for pumping and in certain high-speed rotative engines for electric light and power .service. Ill the latter form of engine two cylinders are used which are coupled to separate cranks on the same shaft in order to secure continuous action. Two familiar forms are the Westinghouse and the Winans, and both are inverted vertical trunk engines. Owing to their single action and the omission of the connecting rod, these engines escape many of the disadvantages of the double- acting high-speed engine. The Cornish single-acting pumping engine de- serves particular notice because of its essentially