Indicator-diagram

Indicator-diagram, a diagram drawn on an indicator-card by the pencil of the indicator of an engine at work. The object in view is to ascertain the relations between, and also the product of the varying pressure and the corresponding variations of volume of, the working substance—steam, explosive gas mixture, hot air, or other material. The latter, the variations of volume, are, in a cylinder, well represented by the movements of the piston; the former, the varying pressure, may be followed by making the steam, &c. press out the piston of a small side-cylinder against the resistance of a spring. If a pencil be attached to this piston it will mark on a piece of paper or card held in contact with the point a straight line traced and retraced with varying velocity. If the steam be shut off from this side-cylinder the pencil assumes the position of 'no pressure.' If now, on the other hand, the piston of the main cylinder be made to draw the paper or card past the pencil point in a direction at right angles to the former, the varying velocity with which a straight line is traced and retraced on the paper will reproduce the varying velocities of the main piston itself. If these two actions be now combined the pencil will move, say, up and down, while the paper will oscillate or be unrolled backward and forward. The pencil-point will accordingly describe upon the paper an irregularly-curved figure which will, in uniform working, be a closed curve, and will always tend approximately to reproduce itself during each successive cycle of the engine. Upon the scales on which the linear traces of the pencil represent, in directions at right angles to one another, the variations of pressure and the piston-movements respectively, the area enclosed by this curve will represent the work done by the engine during each cycle; and its form enables the actual pressures and volumes of the working substance to be traced out for each successive portion of the cycle, and thus enables the working of the engine to be carefully studied in detail. For examples see Holmes, The steam-engine; and Dugald Clerk, Gas-engines. See article GAS-ENGINE for diagrams.