the arrangement would not be economical. For these reasons, all gas engines are arranged to receive their fuel charges a cylinderful at a time, and to ignite, burn, expand, and reject each charge before receiving the next.
20. Representation of Work In a Gas Engine. — In order that what takes place in the cylinder of a gas engine may be better understood, the various processes will be explained with the aid of the pressure-volume diagram as produced by an instrument called an indicator. Fig. 6 illustrates the principle on which this diagram is obtained. Suppose that P is the plunger of a pump or the piston of an engine, working in a cylinder C. In the operation of any engine, there is a more or less constantly varying pressure in the cylinder, and in order to study what takes place within the cylinder it is necessary to provide some instrument that will automatically record the pressure at all points in the movement of the piston. This may be done by means of the indicator, of which it is necessary at present to study only the elementary form shown in the figure.
21. Let the small cylinder c be connected with the interior of the large or working cylinder C. In c is a small piston p to which is attached a spring s. Attached to p, also, is a rod b carrying at its upper end a pencil a. At f is a slide, attached by the arm h and cord e to the arm r, which is fastened to the working piston P. Running in the opposite direction from the slide is a cord g passing over a pulley d and having at its end a weight w.
It will be seen that, as the piston moves back and forth during the operation of the engine, it will carry with it the slide f. As the pressure changes within the cylinder C, it also changes in cylinder c; hence, the piston p moves up or down as the pressure rises or falls. Therefore, the point of the pencil a will be in a position that, measured from left to right, corresponds to the position of the piston P> in the cylinder, measured from right to left, and the height of the. pencil corresponds to the pressure within the cylinder C. As the area of the end of the piston does not change, and since
