STEAM AND STEAM-EXGINB. 



STEAM AND STEAM-EXGINE. 



rin 



it portable, the oumbroui apparatus of the condenser, and its 

 tteaiUnt pump* and cisterns, had to be discarded; and since the 



Fig. 11. 



principle of condensation was consequently renounced, it was 

 Decenary to raue the steam to a pressure sufficient to overcome that 

 of the atmosphere on the opposite side of the piston. To allow the 

 Bteam to act alternately on both sides of the piston, that which had 

 just acted on one side to drive the piston was expelled into the open 

 air through an orifice, corresponding to that which would have con- 

 nected the cylinder with the condenser in an engine of the usual 

 construction ; but unless this orifice were as large as the diameter of 

 the cylinder, which obviously it never can be, the steam, retarded in 

 its escape by the contraction of the passage, must diminish, by its 

 resistance to compression, the effective force of that which is acting to 

 impel the piston. 



Such is the simple principle, and such the greatest defect, of the 

 non-condensing engine ; but the saving in original cost and thr 

 paramount advantage of portability more than compensate for this 

 defect ; so that the use of this kind of engine has become general, not 

 only for the purposes of locomotion, but for a variety of others where 

 the engine is stationary, and probably in many instances where its 

 advantage* are imaginary. 



Since the pumps of the condensing engine are dispensed with in the 

 non-condensing one, the beam may be so likewise ; the piston-rod is 

 made to move in a straight line, by having a cross-piece attached at its 

 top, which slides between guide* fixed on each side of the cylinder, the 

 rod which works the crank of the fly-wheel being attached to the end 

 of this cross-piece. A still further simplification is effected by con- 

 necting the piston-rod directly with the crank on the shaft of the fly- 

 wheel, the cylinder being mounted so as to oscillate as the wtteel 

 revolves on the steam passage, and thus alternately to open and close 

 the communication between the top and bottom of the cylinder. 

 Such engines are termed vibratory, and are successfully used where 

 pace mult be economised, as with marine engines, but the weight of 

 the cylinder thus moved is so much to be deducted from the power of 

 the engine, and further causes a rapid wear of the centres on which it 

 turn*, which consequently cannot be long preserved steam-tight, and 

 require frequent renewal. 



Steam-engine* are properly classed, according to the principle on 

 which the physical properties of the steam are employed in them, 

 into 



1. Condensing Snyintt. 



1. Atmospheric engines, acting by Condensation only. 



2. Double-acting engines . . . Pressure and condensation. 



/Pressure, expansion, and 

 8. Double-acting engines . . . .| condensation.^ 



2. Non-condensing Enginet. 



1. Engines worked by . . . . Pressure only. 



_'. Kngines worked by . . . Pressure and expansion. 



The form of the engine, the arrangement and construction of its 

 part*, it* power, Ac., depend entirely on the purpose to which it in to 

 be applied, and may be indefinitely diversified, but those moat in use 

 may be artificially cUiMed thus 

 1. C'jndtnring engina, with t-camn and jmnill.-l motions. 



1. Without a fly-wheel, for pumping in mines, to. 



2. Marine engines. 



3. With a fly-wheel, for working machinery. 



3. IfoH-cundetwing mgina, without a beam. 



1. Stationary, with a fly-wheel for working machinery. 



'-' Uotatory engine*. 



3. Locomotive engine*, without a fly-wheel 



Marine engines, or those used for propelling vessel*, are in this 

 country generally condensing engines, their situation admitting the 

 abundant use of cold water. The principal peculiarity in the arrange- 

 ment of the marine engine is the position ,,f the beam, which, for the 

 purpose of economising room, is placed lower than the cylinder, and is 

 double, there being one on each side ; a rod from one end of each of 

 these is connected with a cross-piece at the top of the piston-rod, the 

 rectilinear motion of which is produced either by guides, or by a crank- 

 arrangement, analogous in it* action to the parallel motion. The other 

 end* of the double beam are connected by a cross-piece, carrying in ita 

 centre the " rod " to work the crank on the shaft of the paddle*. In 

 all vessels of any magnitude, there are two engines complete, so 

 arranged that while the rod and crank of one are in their n 

 position, those of the other are in that of greatest effect. Two engines 

 are necessary to equalise and continue the motion of the wheels ; for 

 in the marine engine, the paddles, instead of performing the | 

 fly-wheel* to continue and control the motion of the piston, requii 

 whol.' force of the engines to maintain their own motion, owin^' : 

 resistance they have to overcome. There is also this further advantage 

 derived from two engines, that if one should be injured, the vessel 

 may be still propelled by the other, and not be entirely dependent on 

 her sails, as she would otherwise be. 



It has been mentioned that there is a limit to the proportion between 

 the diameter and length of the cylinder; the advantage that would 

 accrue in gain of power by a long stroke being diminished l>y thu 

 greater radiation of heat from the larger surface diminishing tin 

 of the steam in the cylinder ; here therefore, as in every other < 

 tion connected with the steam-engine, it is hardly possible to arrive at 

 any formula or rule that can be invariably used. If the surface of tbe 

 cylinder were to be made a minimum, with a maximum of cai 

 we could readily determine that the length should be twice the 

 diameter ; * but we find that this proportion is not adhered to by the 

 best makers ; it varies from 3 : 1 to 2 : 1 ; but in the marine engine it 

 is usually shorter ; in some instances the proportion is 1 : l"J, r >. 



The diameter of the cylinder of a marine-engine is usually greater, 

 in proportion to its length, than it is in others, in order to obtain, by 

 an increased surface of piston, that power which is unattainable by a 

 long stroke, owing to the limited space which can be appropriated to 

 the engine. Formerly, the apprehension of danger, where so many 

 lives were at stake, prevented the use of steam of more than 4 to 

 6 Ibs. on the inch in marine engines, and the expansion prin 

 quently could not be had recourse to. At present, the economy of 

 using this principle has outweighed the apprehension in the in 

 the owners of vessels, while the public, contented with the informal ii >u 

 that the engine is a condensing one, seldom inquire further, and con- 

 ceive that the steam is at a low pressure in all marine engines, although, 

 where the expulsion principle is used, which it now extensively is, the 

 pressure in the boiler is at about 30 Ibs. on the inch above the pressure 

 of the atmosphere. ! 



Engineers have always been induced, by the obvious advantage of a 

 continuous over an alternating motion, to aim at contriving a steam- 

 engine in which the steam should act directly to produce such a 

 motion. It does indeed appear at first sight that, where the object of 

 the engine is to produce a continuous circular motion of a fly-wheel, or 

 of wheels of some kind, it would be desirable that the steam should be 

 applied directly to impel the wheel, instead of having its force < 

 mitted through a series of levers, necessarily increasing the friction and 

 the cost of the engine. Watt accordingly patented more than one of 

 such rotatory engines, and many others since have from time to time 

 brought forward arrangements for the purpose, but none have come 

 into permanent and general use. The fact is that, as can be easily 

 shown, the employment of steam in this way is productive of a greater 

 waste of power, with a greater increase of friction, than can be com- 

 pensated by any real advantages. In all rotatory steam -engine* 



* Let / = length, x = the diameter, c = the capacity of the cylinder ; since 

 the concave surface is only gradually brought into contact with the steam, by 

 the motion of the piston, its effects on the temperature may be considcrnl as 

 about half what it would be if the whole surface were at once exposed. Then 

 the whole surface, including the two ends, being 



* fix* 



irxI+3 andc= , 



wo have for the surface affecting the temperature of the steam 



therefore 



2c 



ted* 



and by substituting the value of c, and reducing lx = l. 



t On the Mississippi the boat-engines are worked with -ti'.mi of from 10 

 to ISO Ibs. on the inch; but the latter mormon* pressure is rarely exceeded, 

 " cxofi't " as an American cymuiuiiucr saidj *' on fjtrtwrtlitia~" 



