THE STEAM-ENGINE. 



simply means pressure less than the atmospheric 

 pressure; and, in the case of steam-engines, a 

 vacuum generally implies a pressure of between 

 two and four pounds per square inch ; that is, from 

 a seventh to a fourth of the ordinary pressure of 

 the air. The most common way of condensing 

 steam is by bringing it into contact either with a 

 jet of cold water, or with surfaces kept continually 

 cool by a current of water. In either case, 

 directly the steam is brought into contact with the 

 water, or cooling surface, it transfers to it the 

 larger portion of its sensible heat. During this 

 process, the greater part of the steam is liquefied, 

 and the remainder retains only such a pressure as 

 corresponds to its greatly reduced temperature. 



The advantages possessed by a condensing 

 over a non-condensing engine will now be obvious. 

 When the piston is being forced from C to D by 

 steam entering through C, the force on the back 

 of the piston resisting its motion in that direction, 

 instead of being equal to the pressure of the 

 atmosphere, is only the pressure of the steam in 

 the condenser, or about i Ib. per square inch. 

 The net effective force is therefore 60 i, or 59 

 Ibs., instead of 60 15, or 45 Ibs. In actual 

 practice, these figures would be modified, because, 

 from various causes, such a low back-pressure as 

 i or 15 Ibs. above zero (in condensing and non- 

 condensing engines respectively) is never obtained, 

 but the principle remains the same. 



We have supposed that our cylinder when full 

 of steam contained just i Ib. weight at 60 Ibs. 

 pressure. Let us now find out how much useful 

 work this pound of steam has done for us, and we 

 will then shew how the same weight may be made 

 to do a great deal more, by utilising more of its 

 great store of heat Let us suppose that the area 

 of the cylinder is 2 square feet, while its length 

 (the stroke of the piston) is 3^ feet. It will thus 

 have a capacity of 7 cubic feet, as before assumed. 

 In the first case described, we should have a 

 pressure of 45 Ibs. per square inch exerted on an 

 area of 288 square inches through a distance of 

 3| feet. This is equal to 45,360 foot-pounds of 

 work. In the second case, we have a pressure of 

 59 Ibs. per square inch on the same area, and 

 through the same distance. This is equal to 

 59,472 foot-pounds of work, or about T Vth of the 

 total heat supplied by the fuel.* We may now 

 proceed to examine the way in which the same 

 weight of steam, generated by the consumption 

 of an identical weight of fuel, may be made to 

 perform many times more work by 'working 

 expansively.' 



We have already said that one of the properties 

 of steam is a tendency to expand indefinitely. 

 We also mentioned and explained the law that 

 its pressure varies inversely as its volume. We 

 will now describe the way in which this is taken 

 advantage of by the engineer. If we have a 

 cylinder of the same area as before, but of twice 

 the length, but only intend to admit one pound of 

 steam into it at a time, it will be necessary, when 

 the piston has travelled 3^ feet of its stroke, to 

 shut the entrance valve, so as to prevent more 

 steam entering; this is called 'cutting off' the 

 steam. The piston, however, still continues its 



For simplicity's sake, we have here assumed that the water in 

 the boiler has to be raised from 33 to 292*, and evaporated at that 

 temperature. If the water were supplied at aia*. then the work 

 done would be about t^th instead of Ath of the total heat 



motion in the same direction as before, propelled 

 by the internal separative energy among the 

 particles of steam. But as it is pressed forward, 

 the space occupied by the steam is always in- 

 creasing, and its pressure always decreasing in 

 proportion, until at length, when the piston has 

 reached the end of its stroke, the steam occupies 

 exactly double its original volume namely, 14 

 cubic feet, and is reduced in pressure to half its 

 original pressure namely, to 30 Ibs. per square 

 inch. We have thus during the first half of the 

 stroke a constant pressure on the piston of 60 Ibs. 

 per square inch, and during the second half a 

 pressure gradually decreasing from 60 to 30 Ibs. 

 The mean pressure during this second half of the 

 stroke will be found on calculation to be almost 

 exactly 40 Ibs. Let us now in the same way as 

 before see what work we have been able to get 

 out of our pound of steam by expanding it in this 

 way. In the first half of the stroke we have 

 59,472 foot-pounds of work exactly as before, and 

 then we have in addition a mean pressure of 40 I, 

 or 39 Ibs. per square inch exerted over 288 square 

 inches for a distance of 3^ feet. This equals 39,312 

 foot-pounds, making a total of 98,784 foot-pounds 

 of work obtained from the steam which only gave 

 us 59>47 2 before. The economy of working ex- 

 pansively, however, goes much further than this. 

 If the cylinder had been four times its original 

 length, and the steam had been cut off at the 

 same point as before (which would then be quarter 

 instead of half stroke), we should have obtained 

 from the i Ib. of steam 144,345 foot-pounds of 

 work. If we had gone still further, and expanded 

 the pound of steam into eight times its original 

 volume, we should have obtained no less than 

 179,984 foot-pounds of work, which is more than 

 three times as much as at first.* All modern 

 engines are worked more or less on this principle 

 of expansion, and the general tendency seems to 

 be every year to adopt higher initial pressures, 

 and larger ratios of expansion. 



Fuel, We must now go back a little, and con- 

 sider the relative qualities of different fuels, and 

 the economical effects produced by their combus- 

 tion. The principal constituents of all substances 

 used for fuel are carbon and hydrogen. They 

 mostly contain also a small amount of oxygen, and 

 a considerable percentage of incombustible matter 

 called ash. Speaking generally, that fuel is the 

 best which contains most carbon, and least of the 

 other constituents. The great heat developed by 

 the combustion of coal is the result of the chemi- 

 cal combinations which occur during that com- 

 bustion, for it is now a well-ascertained fact that 

 all bodies in the process of chemically combining 

 with others liberate heat, just as in decomposing 

 they absorb it 



When coal undergoes combustion, the action 

 which takes place is as follows : The fuel is first 

 decomposed into two parts, .solid and gaseous ; 

 the former remains on the grate as incandescent 

 coke, and gradually combines with the oxygen of 

 the air ; and the latter consists of free hydrogen 

 and the hydrocarbon gases. The hydrogen in 

 these gases, having a much greater affinity for 

 oxygen than for carbon, quickly rids itself of the 



* In actual working, owing to various causes such as im- 

 perfect action of the valves, radiation from the cylinder, bad 

 vacuum, &c. the work obtained from the steam is not more than 

 65 to -75 of that given in this paragraph. 



