All About Engines

All About Engines the illustration. Any movement of this piston operates a lever which carries on its right-hand end a small pencil or pen. The paper is mounted on a drum which has a spring inside. On a pulley near the lower edge of the drum a cord is fixed and wound round for several turns. The other end of the cord can be hooked on to a lever that is moved backwards and . ■°° __ forwards by the cross- ly I head, or, in the case of \ a gas or oil engine, by I« x. \ the piston. This lever K —-------------——has arms of such lengths Fig. 177,-Indicator diagram of that a Complete Stroke steam engine £ , oi the engine causes a movement of the paper of, say, 3 inches. This ratio is a definite one. If the tap connecting the indicator is closed and the engine is working, the drum is turned a definite portion of a revolution when the piston moves forward, and returns to its original position, under the influence of the internal spring, when the piston moves back again ; and each time the pencil draws on the paper a straight line which represents the length of the stroke to a definite scale. Similarly, if the drum is disconnected and the tap is opened, the movement up and down of the pencil represents to scale the variations of pressure within the cylinder. When the drum is connected and the tap opened for two strokes of the engine, both vertical and horizontal movements take place at once, and we get a closed diagram like that in Fig. 177, which shows exactly to scale the variations of the