There have been several systems proposed by which the gas in the cylinder is withdrawn by the inertia of the exhaust gas in specially designed ejectors, and by the compression of fresh air in the crank case of the engine. The former system known as “organ pipe ejection,” is by far the simplest method of all as the ejector is simply a tube without moving parts, and it also possesses the additional advantage of reducing the back pressure on the piston. Unfortunately these advantages are obtained only at certain loads, and with certain velocities of the exhaust gases, which makes it impossible to obtain even approximately correct scavenging at other loads and speeds. When air pumps are used for scavenging, a great percentage of the economy obtained is offset by the power required to operate the pumps. In addition to the frictional losses of the pumps, are the increased maintenance charges and repair bills. CHAPTER IV INDICATOR DIAGRAMS (35) General Description. A brief description of the indicator as a means of recording the pressures in the cylinder of a simple heat engine in relation to the piston position was given in paragraph (6), Chapter I, and as this instrument is so peculiarly adapted to locating the events taking place in the cylinder we will devote some space on its application to the practical gas engine cycles described in the preceding chapter. Since each event in the cycle is accompanied by a corresponding increase or reduction in pressure, the beginning or end of an event will be indicated on the diagram by a change in the vertical height of the curve above the atmospheric line, at some particular piston position. The piston position will be in the same relation to the total stroke as the pencil position will be to the horizontal length of the card. If the event, for example, as indicated by a drop in pressure, be at the center of the card, it will show that the drop in pressure took place when the piston was in the center of the cylinder or at mid-stroke. Should the pressure change at a point one-quarter of the card length from the starting point of the pencil, it shows that the event took place in the cylinder when the piston had accomplished the first quarter of its stroke, and so on. It should be noted that horizontal distances on the indicator card denote piston positions, and the vertical distances, pressures.