In further elucidation of the subject, a few diagrams (reduced) are added, together with some practical hints as to the important information to be obtained from them. Fig. 37. A 66 " The "steam corner" A, or commencement of stroke, being very angular, shows that the valve is too forward at the beginning (see page 320). AB being near parallel to the atmospheric line, shows that there was no expansion, but supply of steam to the end of the stroke with pretty equal pressure. The "exhaust" or eduction corner C should be more nearly square and not rounded off, the action as here indicated is not very good, as the exhaust valve opens too late, therefore, power is lost, because the steam does not get into the condenser soon enough. D is the lead corner, and is too square, and shows that the valve opens too late. When the lead corner D is much slanted off, the amount of lead is great (see also Fig. 45); but the cutting away of the lead corner, to a small extent, is not to be looked upon as a defect. (See page 320). Fig. 38. This is a fair average diagram. The fulness of the part below the atmospheric line shows that a good vacuum was obtained; the depression at B shows that the steam was cut off there at about two-thirds the whole stroke of the piston (as measured by xy); hence, from B to C the steam was acting expansively. At A, however, it shows that, if not caused by the friction of the piston of the indicator itself, there was too sudden a rush of steam, which is trying to the machinery from its abruptness. L Fig. 39. This shows an inclined supply steam line, with full expansion, exhaustion, and undulated lead. It illustrates the imperfections of the gear used for the supply of the steam; if the pressure in the boiler were constant and the gear perfect, the dotted lines indicate the formation that should have occurred. Fig. 39. Fig. 40. a с Fig. 40. This engine has small steam ways, as shown by the smallness of a, the steam side; the vacuum side is bad, for there is much steam uncondensed, it causes also noise in the cylinder from the passages being too small. The exhaust valve opens too late, hence the steam does not leave the cylinder at proper time (see at ¿); there wants "lead" on the exhaust side; hence waste of fuel and considerable loss of power. Fig. 41. In this instance the steam is cut off at half stroke, at the end of which there was (as shown at a) some steam to condense; the fulness of b shows there was a good vacuum. Fig. 41. a b Fig. 42. Fig. 42. Steam cut off at one-third stroke, b is too much rounded. Exhaust opens too soon, hence loss of power. Fig. 43. In this case the slide-valve requires more lead and more lap. The exhaust opens too late and closes too late. The steam ways are too small altogether. The vacuum is very good, but too rounded at the exhaust corner. Fig. 43. Fig. 44. ¿ Fig. 44. In this case a shows that the valve has no lead, and that the pressure of steam strikes the piston with some force, to the great strain of the engine, by its suddenness; the part 6 shows that the cut-off was not good, the steam was wire-drawn, "entering after the cut-off," and, indeed, the wavy lines of the figure indicate great derangement of the valves generally. Fig. 45. In this figure a, by being too much rounded and hollowed, shows that the valve has too much lead; the cut off at 6 shows expansion; the exhaust opens rather early; it is, however, a pretty good diagram. a Fig. 45. b Fig. 46. Fig. 46. In this instance the steam corner is slanted off, the steam being admitted too late to the engine (see page 320); the exhaust is too soon. Valves out of order. The rounded corner at a shows cushioning, caused by the too early closing of the eduction-valve before the end of the stroke, whereby steam is shut in and compressed by the piston. Fig. 47. This is a very fair diagram, steam cut off at first grade; a was caused by friction of the indicator piston. Fig. 48. In this diagram a shows too much compression or cushioniong, owing to the slide-valve closing the exhaust too soon, occasioned by having lap on the exhaust edge of the valve, or in working with the link not in full gear. 300. Top and bottom Diagrams exhibited on the same card. It has been already stated that the indicator may be attached to a pipe connected with the top and bottom of the piston, and thus both the diagrams may be taken on the same card without shifting the instrument. As this is the plan usually adopted at present, a specimen of the two diagrams is given, which is as follows: Fig. 49. It will be observed that the two figures face each other, as they evidently ought to do. 301. Where accuracy is required, a diagram should be taken from the top and bottom of the cylinder. The diagram taken from the top of the cylinder shows only the pressure and vacuum on the upper side of the piston, and, therefore, cannot indicate what is going on below the piston. If our object be merely to calculate approximately the horse-power of the engine, and it be in tolerable good working condition, it is not of much consequence whether the diagram be taken from above or below; but, if the horse-power is required with any great accuracy, the mean result obtained from top and bottom diagrams must be used. It will often, and perhaps most generally, be found that two diagrams taken at different ends of the cylinder will in some respects be dissimilar. In estimating the horse-power of the engine, it is proper that this difference should be known and the average of the two taken. If the actual state of the engine is required, it is necessary to examine into what is passing both above and below the piston, because the errors in one part may have no connection with the errors in another. This will be the case if the slide is too long or too short, in which case the upper part may be properly covered, and the lower one not so; or the upper slide-face may be steam-tight and the lower one leaky; but, if the indicator be applied to top and bottom, it will detect all these inaccuracies, and prevent our attempting to improve the working of one side to the detriment of the other. It ought to be remarked here, that in direct-action engines, the diagram from below the piston is generally superior to the other. First, because, since the steam has more work to accomplish, the piston does not run away from the steam so readily, and in consequence, the steam pressure is better maintained ;* and there is generally a little more lead to the slide, to allow a freer ingress to the steam. There is also another reason why the one diagram is better than the other. The eccentric and crank are so connected that they revolve together, and on account of the smallness of the throw of the former compared with the length of the eccentric-rod, if the steam be cut off when the crank has described a certain angle from the top centre, it will, if the lead be the same on both ports, be again cut off when it has described the same angle from the bottom centre, but because of the shortness of the connecting-rod, the piston will not have traversed so far when the crank has descended through a given angle as it will when it has ascended through the same angle. For instance, when the crank is horizontal and descending, the piston will not be half-way down; but, when horizontal and ascending, it will be more than half-wap up; and, hence, as the crank moves from the top centre, the space traversed by the piston till the steam is cut off is less than when moving from the bottom centre. It therefore follows, that in direct-action engines the steam line is continued further in the up stroke than in the down stroke, and the reverse will be the case in beam engines; and this difference becomes more apparent as the connecting rod is shorter in comparison with the crank. 302. To calculate the mean pressure on the piston throughout the stroke from an indicator diagram. For the purpose of ascertaining the condition of the valves, piston, &c., it will, in the hand of an experienced engineer, be quite sufficient for him to see the general outline of the diagram; but if it is to be used for ascertaining the power of the engine, its average measurement is to be ascertained. This will be be best done by RULE CXXXII. 1o. Divide the diagram in the direction of its length into ten equal parts by drawing nine ordinates, or lines perpendicular to the atmospheric line at equal distances.† * The difference in the speed of the piston at the opposite ends of the cylinder is, at the outer end of a direct-acting engine, from one-sixth to one-third greater than at the crank end-the difference varying according to the degree of angular vibration of the connectingrod. In side lever or beam engines these proportions are reversed, and the speed of the piston is greater at the upper end of the cylinder. + Dividing a diagram into ten equal parts is a somewhat slow process by rule and compass, and an ingenious contrivance has been introduced with Mr. RICHARDS' indicator for this purpose. It consists of a parallel rule composed of eleven bars of thin steel, and two strips; and by bringing the edge of the first bar against a perpendicular drawn on the atmospheric line, the contrivance can be moved so that it contracts so as to be equal to the length of the diagram, whatever that may bo, and an accurate division into 10 equal parts is made with surprising facility by drawing pencil lines against each of the bars. NOTE.-In practice 10 is the usual number of parts employed (see figure following) but any number may be made use of; and the greater the number the more correct will be the result (see Fig. 57). The advantage of taking 10 ordinates instead of 8, 9, or II, is, that the division by 10 is accomplished by merely shifting the position of the decimal point, while 10 ordinates are enough to enable the area to be measured accurately enough for all practical purposes. 2o. With the scale to which the indicator is constructed, measure between the spaces the distance from the atmospheric line to the upper outline (or the steam side) of the diagram until this crosses the former, if it does so. Next, repeat the process for the area between the atmospheric line, or the expansion curve, after it has crossed this line and the lower outline of the diagram. (See Fig. 50 and 52 in illustration.) 3°. Take the sum of the measurements of the steam side, then of the vacuum side, and divide by 10 in each case. 4°. Add vacuum to steam which gives the total mean pressure for each part of the stroke. In finding the mean effective pressure off the diagram it is not at all necessary to take the steam and vacuum effects separately. The usual and most expeditious mode is to proceed thus:-* (a) Take measurements at once completely across the diagram, that is, measure between the spaces from the vacuum line to the steam line; note these scale distances down. (b) Add the whole together and take the mean by dividing by 10, the result is the total mean pressure. In the case of a double diagram, or diagrams from top and bottom of cylinder combined, proceed as directed in Nos. 2o, 3o, and 4°; or as directed in 4° (a) and (b), then, 5°. Lastly, add top and bottom together, and divide by 2, the result is the average of both, or the mean pressure for the whole double stroke. NOTE.-The mode of admeasurement as given in paragraphs 2o, 3o, and 4°, of the foregoing rule is not a strictly accurate one for ascertaining the power of any condensing engine, the steam line of which expands below the atmospheric or datum line. The method, however, does not give a really wrong result when it is wished to ascertain the gross power of an engine by means of the indicator; if, however, it be desired to find separately the respective values of the steam and vacuum pressures, the method is decidedly incorrect. The gross pressure on the piston throughout the stroke is, by this method, obtained at the expense of double work-it not being at all necessary to measure separately the ordinates of the steam and vacuum curves. In any properly constructed condensing engine the steam line of the diagram expands below the atmospheric line. At the beginning of the stroke it is above the line, and further on it is below the atmospheric line. In adding up the steam pressures, only This is the method that is always adopted by the steam department of the Admiralty, and the principal firms in the kingdom. |