Fig. 50. A Typical Indicator Diagram
The indicator card obtained in this way is shown in [Fig. 50]. It is sometimes preferred to take the diagrams of the two ends on separate cards, but it is simpler to take them both on the same one, and also easier to compare the working of the two ends of the cylinder.
Fig. 51. Diagram for Illustrating Method of Computation
The analysis of a card for practical purposes is shown in [Fig. 51]. Suppose, for example, that the length of the diagram measures 3.6 inches; the distance to the point of cut-off is 1.2 inch; and the distance to the point of release is 3.3 inches. Then, by dividing 1.2 by 3.6, the cut-off is found to occur at 1.2 ÷ 3.6 = 1⁄3 of the stroke. Release occurs at 3.3 ÷ 3.6 = 0.92 of the stroke. Compression begins at (3.6 - 0.5) ÷ 3.6 = 0.86 of the stroke. The diagrams shown in [Figs. 50] and [51] are from non-condensing engines, and the back-pressure line is therefore above the atmospheric line, as indicated.
The indicator diagram gives a means of determining the mean effective pressure, from which the power of the engine can be found from the previously given equation
| APLN | |
| I. H. P. = | ———. |
| 33,000 |