APPLIED MECHANICS. 



[INDICATED HOMB-FOWE*. 



If we nippose that the area of the indicator-piston is 

 1 square inch, and that the spring is adjusted so that it 

 requires a force of 10 Ibe. to oomprem it 1 inch in length, 

 or 1 Ib. to compress it .foth of an inch, we can form a 

 scale of tenths of inches, and apply it to the indicator 

 diagram at a number of point*, A, B, C, <to. (Fig. 200), 



MO. 



equally distant ; anil measuring offtho lengths of ordinates 

 A L, B M, <fec. , drawn through these points, we can thus 

 estimate the pressures acting on the piston of the indi- 

 cator-cylinder at equidistant points of the stroke through 

 which the paper is made to travel. These pressures cor- 

 respond exactly, per square inch, with those to which 

 the main piston of the engine is subjected during its 

 stroke, because the small cylinder of the indicator com- 

 municates freely with the cylinder of the engine. If we 

 suppose the indicator to bo fixed at the top of the cylinder, 

 the upper part of the curve a, L, M, <fcc., B, Z, is that 

 traced during the descent of the piston when the steam 

 is pressing on it. The lower part of the curve Z, Y, ibc. , 

 8 a, is that traced during the ascent of the piston when 

 the steam is escaping from the cylinder. Were the indi- 

 cator fixed to the bottom of the cylinder, we should get 

 corresponding curves for the steam -pressures there. 

 Generally, when the eccentric-slide and its levers are 

 properly adjusted, these figures are very nearly alike ; 

 and, if so, the upper part of the curve may be taken as 

 that traced by the active pressure either above or below 

 the piston, while the lower part of the curve may be 

 taken as representing the corresponding resistance of 

 steam during its egress from the cylinder. Now, as the 

 total height C N of any ordinate, measures the total 

 pressure on one side of the piston when it is at the 

 point of its stroke corresponding to C ; and as the part 

 C U of the same ordinate represents the resisting pres- 

 sure on the opposite side of the piston at the same point 

 of its stroke ; the difference U N, or the part of the ordi- 

 nate intercepted between the upper and lower limbs of 

 the curve, measures the effective pressure on the piston 

 clear of all resistance. The same applies to all the ordi- 

 nate* ; and, as wo may suppose, the whole curved space 

 made up of numerous equal, narrow, vertical strips, each 

 measured in height by an ordinate, we may reckon the 

 area of the figure contained within the curve as an ex- 

 pression of the power developed by the piston during its 

 stroke. Or, having taken a considerable number of these 

 pressure- ordinates, and found their average, we may con- 

 sider this the mean effective pressure on the piston. For 

 example, the average of those marked in the figure, found 

 by adding them into one sum, and dividing it by their 



number, is .. _ 20 Ibs., the mean effective pressure on 



every square inch of the piston. In taking the average 

 in thin way, the most correct method is to take the first 

 and last ordinates, A L and G R, at distances, K A, H G, 

 from the respective ends of the stroke a and Z, half of 

 A H or B C the distance which separates the other ordi- 



If we suppose the engine from which this figure was 



taken had a cylinder 12 inches in diameter, a stroke of 



5 inch, and thnt the crank made 80 revolutions per 



minute, we can readily calculate the effective power of 



the engine thus : Each revolution of the crank requires 

 an up-stroke and a down-stroke of the piston, or a travel 

 through twice 15 inches, viz., 2} feet ; and as 80 revolu- 

 tions are made per minute, the piston travels over 

 80 X 24 =- 200 feet per minute. Again, the area of 

 the piston 12 inches diameter is 113 square inches, and 

 as this is pressed on with an average load 

 of 20 Ibs. on every square inch, the total 

 pressure on it is 113 X 20 - 22GO Ibs. 

 We have, therefore, 2200 Ibs. moved 

 over 200 feet per minute, equivalent 

 to 2260 X 200 = 462000 Ibs. moved 

 over 1 foot pel minute, which give 



452000 



- 13$ horse-power nearly. 



The complete rule for finding the 

 power may therefore be thus stated : 

 Measure the ordinates (at least C or 8 

 in number) contained within the. indi- 

 cator figure, sum them up, and divide 

 by their nnmber for the mean pressure ; 

 multiply the area of the piston (in 

 nquaro inches) by the mean pressure (in Ibs. per square 

 inch) by twice the length of stroke (in feet) and by the 

 number of revolutions per minute, and divide the pro- 

 duct by 33,000 for the horse-power. 



Example. On the indicator being applied to an en- 

 gine, having a cylinder 30 inches diameter, a stroke of 

 4 feet, and making 27 revolutions per minute, 8 onli- 

 nates of the figure were found to be respectively 34, 34, 

 34, 33, 24, 18, 14, and 9 : required the power of the 

 engine. 



Sum of 8 ordinates = 200, which, divided by their 

 number 8, gives 25 Ibs. as the mean pressure. 



Area of cylinder, 30 ins. diameter = 707 square inches. 



Multiply by mean pressure . 



Total mean pressure on piston 

 Double the stroke 





Number of revolutions 



17f,7.-.lbs. 

 8 feet 



141400 



27 per minute. 



33000) 3817800 



. 115} nearly. 



Divide by . 



Horse-power . 



But in making these calculations, it must not be for- 

 gotten that we only reckon the force with which the 

 piston moves. In communicating this force to the 

 crank-shaft, and thence to the machinery driven by the 

 engine, there are losses from friction and other causes 

 for which some allowance must be made. The piston 

 rubs along the surface of the cylinder ; its rod rubs 

 through the stuffing-box ; so with the slide. The end of 

 the piston-rod rubs on the guides, which save it from 

 yielding to the oblique action of the connecting-rod ; the 

 connecting-rod eyes rub on their pins, the crank-shaft 

 rubs in its bearings, the eccentric and its rod also pre- 

 sent rubbing surfaces, the fly-wheel encounters con- 

 siderable resistance of the air to its rotation, and the 

 feed-pump demands power for its working. All these 

 resistances vary with the conditions of the nibbing sur- 

 faces, the accuracy or inaccuracy of their fitting, their 

 state of lubrication, and other circumstances ; so that it 

 is difficult to state any constant deduction to be made 

 from the calculated power on account of them. An 

 engine in a very good state should thus waste not more 

 than -,\jth to jth of its power ; while one in a bad state 

 may lose as much as Jrd. It may generally be fair to 

 reckon the loss at 4th or Jth of the calculated power. 

 Thus, in the example given, the calculated power being 

 116J, we take the real power about 90, deducting rather 

 more than Jtli for losses. 



The indicator figure is not only a measure of power, it 

 is also a picture of defects, and may often furnish useful 

 hints as to the proper mode of improving the action <>f 

 the engine. If A U (Fig. 201) represent the stroke, 



