18420 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



Ill 



Fig. 3.— Indicator di»gr»m of Sm»ll Cylinder. 



Fig. 4. — Indicator diagram of Large Cylinder. 



Performance of Esuine. 



The engine makes 19 double strokes 

 per uiinu^e. 



The consumption of fuel is 132'3 lb. 

 of unscreened Graigola coals per hour. 



The area of the smaller cylinder, le»B 

 i the rod, is 1H6-24 square inches. 



The area of the larger cylinder, lesi 

 i the rod. is 74y-i;y square inches. 



The iiumU-r of feet per minute travel- 

 led by the jniallrr piston is 171. 



The nvmiber of feet per minute travel- 

 led byllip larger piston is 2'2b. 



The mean pressure in the smaller 

 cylinder urging the piston, as shown by 

 tiie indicator, is 2.>'I>o lb. upon the square 

 inch. 



The mean pressure in the larger cylin- 

 der urging the piston, as shown by the 

 indicator, ist)-9 lb. per'quirre inch. 



We subjoin indicator diagrams, taken 

 from both cyiinders.in the oidinary work- 

 ing condition of the engine. 



VVe are informed by Mr. G. Rcnnie 

 that the nominal power of the engine is 

 40 horse power, but that the actual 

 work performed, or power exerted is — 

 By the smaller cylinder . . . 24-6 

 By the larger cylinder .... 3j*U 



Real number of horses power of en- 

 gine fitJ'2 



It is right, however, that we should 

 investigate this point for ourselves. 



The area of a surface included within 

 the lines of an indicator diagram is a cor- 

 rect representation of the power really 

 exerted by an engine, but the mode of 

 finding a nuineriral expression for that 

 area by means of ordinates is not per- 

 fectly correct : although if there be a 

 sufficient number of ordinates, it may be 

 rendered sufficiently correct for practical 

 purposes. \Ve shall, therefore, take the 

 mean pressure in the smaihr cylinder at 

 2Sv"i(i pounds, and the mean pressure in 

 the larger cylinder as (j-i> pounds, as de- 

 termined bv Mr. Rennie. 



18G-24 X 25-5ti X I'l = 

 814H 10-3424 — 3oOtX) = 24-G 



749-2yX G- 9 X 228 = 

 11787S2-G28 -;- 33000 = 



35-7 

 GO-3 



Deduct for friction, pumping 

 water out of condenser, &c., 

 one-eighth 



Real number of horses power 



A L, Atmospheric Line. 



It is easy to determine analytically the amount of advantage that is 

 to be derived by using steam expansively. Let the full pressure of 

 ■team upon the piston be represented by unity, and let x represent 

 the capacity of the larger cylinder, or the space through which the 

 piston has been moved, whilst urged by the expanding steam. The 



density will then be --— and assuming that the densities and elas- 

 tic 

 ticities are proportionate, — — will be the differential of the eflSciency, 



and the efficiency itself will be the integral of this, or, in other words, 

 the Napierian logarithm of the denominator. Wherefore the efficiency 

 of the whole stroke, whether the expansion is accomplished in one or 

 in two cylinders, will be l-f- Nap. log. 1+r. 

 Taking the pressure of the atmosphere at 15 lb., and the pressure 



upon the piston at the commencement of the stroke at 25 lb. above the 

 pressure of the atmosphere, then 15-|-2.5=40 lb. If the steam be cut 

 off at one-fifth of the stroke, or if, as in the present case, the smaller 

 cylinder be one-fifth of the capacity of the larger cylinder, the ste.im 

 will be expanded into 5 times iU original volume. Its pressure, there- 

 fore, at the termination of the stroke, will be ♦,"=8 lb., or 7 lb. below 

 the pressure of the atmosphere ; but as the average vacuum in the 

 cylinder of an engine rarely much exceeds 24 in. of mercury, or 12 lb. 

 pressure, the unbalanced force of the steam, or the force of steam 

 urging the piston at the end of the stroke will be 12-8=4 lb. When 

 the steam is cut off at onc-fiflh .r-^4. The efficiency, therefore, is 

 1-1- Nap. log. (1-1-4) = 1+ Nap. log. 5. 



The Napierian logarithm of S is 1-G00i373, therefore the toLil effi- 

 ciency of the steam is 2G09137?, instead of 1. lu oth.r words, the 



