stroke. In Fig. 17 is shown this type of engine, with the piston on the suction stroke just at the point where cut-off takes place. In this figure, the parts of the engine are lettered the same as in Figs. 8 to 11 and the indicator diagram is shown of the same length as in Fig. 8. It resembles the one in Fig. 15, except that it is shorter. This diagram is an exact copy of one taken during a test of an automatic cut-off engine, with the engine cutting off, that is, the inlet valve closing, at about six-tenths of the stroke. It should be noted that the suction line v drops slightly until cut-off takes place at c, and the continuation of this line in v' drops more rapidly toward the end of the stroke. This is caused by the movement of the piston beyond the point of cut-off, which produces a partial vacuum in the cylinder. On the return stroke of the piston, the compression line is practically identical with the line v' to the point c, crossing the atmospheric line OV near c, and then completing the compression, as in the ordinary four-stroke cycle, as if the stroke extended only from c to O. Ignition takes place at f, and the combustion line fgh is practically identical with the ordinary four-stroke cycle, except that, because this engine is usually made with a water-cooled piston, the compression may be carried much higher. The low pressure of the exhaust, as shown at j, is quite noticeable. The gain over the ordinary four-stroke cycle is shown in that portion of the card beyond the dotted curve j'. The exhaust line w is similar to that of the ordinary four-stroke cycle.
47. The series of operations in the automatic cut-off engine may be tabulated as follows:
AUTOMATIC CUT-OFF ENGINE
First Revolution
First Stroke.— Suction to cut-off; pressure falls slightly; inlet valve open until cut-off; expansion of mixture; pressure falls considerably.
Second Stroke.— Compression; ignition; combustion; pressure rises slowly to point of cut-off, then more rapidly to ignition; both valves closed.
