The area is therefore the mean ordinate multiplied by the length L., Hence, since the mean ordinate represents the mean pressure, and the length of the diagram represents the distance traversed by the piston, the work done on the piston is represented by the area of the diagram. This may also be stated as follows: In any pressure engine, the work performed on the piston is directly proportional to the area of the indicator diagram. It is evident that, since the area of the diagram represents work, its area may be expressed in foot-pounds of work.
FOUR-CYCLE ENGINE
PRINCIPLES OF OPERATION
28. Definitions of Cycles. — The series of operations that produces a complete indicator diagram, as shown in Fig. 6, so that the indicator pencil returns to the starting point, is known as a cycle. In some gas engines, the complete cycle is produced by four strokes of the piston, or two revolutions, and these are known as four-stroke cycle, or simply four-cycle engines. In other engines, the cycle is produced by two strokes of the piston, and these are known as two-stroke cycle, or two-cycle, engines. The cycle is generally considered as beginning at the commencement of the suction stroke, that is, the stroke on which the fuel and air are taken into the cylinder, and to end just before the next suction stroke begins.
29. Development of Four-Cycle Principle. — In the first successful gas engine, the charge drawn into the cylinder under atmospheric pressure during part of the outward stroke was ignited when the piston had traversed about four-tenths of its stroke, the sudden rise in pressure due to the explosion of the gas being utilized to drive the piston to the end of its stroke, work being performed during the expansion of the hot gases. During the return stroke, the burned gases were driven from one end of the cylinder while a fresh charge was drawn in and ignited at the other end,
