Indicator Diagrams

Indicator Diagrams, in steam- and gas engines, are curves drawn mechanically, to show the varying pressure of the steam or gas in the working cylinder throughout the stroke. The instrument by which the curve is drawn is called the indicator; it was first employed by Watt, and has been of great value in testing the efficiency of engines and in pointing out defects of arrangement. It consists of a small, strong, hollow cylinder fitted with a piston. This has a piston rod passing through a cover, and the lower end of the cylinder is put in communication with one end of the steam-cylinder itself by means of a brass or copper tube. Thus, the small piston is subjected on one side to the varying pressure of the steam in the main cylinder. To prevent its immediate passage to the end of its short stroke, the further side of the piston is fitted with a strong spring, of which the exact pressure requisite for each inch of compression is accurately determined. The end of the piston-rod thus jerks up and down with a motion corresponding exactly to the variations in steam-pressure during each stroke, excepting in so far as oscillations of the indicator spring mask the true movements required. Connected with the end of the piston-rod, and capable of magnifying its movements by means of suitable levers, is a small pencil of lead or other convenient material. It presses lightly against a drum, round which a sheet of paper is fixed; the drum is made to oscillate about its axis with a motion corresponding to that of the piston of the main cylinder. If the drum be disconnected from the engine, the recording pencil simply marks a vertical line in a jerky fashion. If the indicator cylinder be shut off from the engine and the drum be connected up therewith, the pencil marks a horizontal line round .the paper oil the drum. If both mechanisms are connected up, the pencil draws a closed curve for each cycle of pressure-changes on the one side of the main piston. The area of the curve measures to scale the energy supplied by the expanding steam (or gas), and with a knowledge of the number of such strokes per minute the indicated horse-power of the engine is calculated.