37. In the accompanying, diagram, Fig. 12, the four strokes needed to complete the Otto cycle of operations are indicated by the spiral around the crank-shaft. The points of valve opening and closing, ignition, etc. are indicated on the spiral by letters, and similar letters with the subscript 1 show the corresponding positions of the piston. The indicator diagram above the cylinder shows the pressure in the cylinder for each position of the piston, and the reference letters with the subscript 2 on the diagram correspond to those on the spiral.
Starting with the piston at a1 and the crank at a, the suction stroke continues until the crank reaches b and the piston b1, at which point the inlet valve closes. The suction created by the rapid movement of the piston produces a slight vacuum in the cylinder, which is shown, exaggerated for clearness, by the drop in the dotted curve, from a a2 to b2, below the line OV, which represents atmospheric pressure. With the inlet valve closed, the piston starts back, compressing the mixture to the pressures corresponding to points along the line b2c2 . With the crank at c, and the piston at c1, the charge is fired. An abrupt rise in pressure follows, shown from c2 to d2 , most of the charge burning during the momentary dwell of the piston near the dead center, or end of the stroke. Combustion being complete and the piston moving out, the pressure begins to fall at d2, the crank being at d. When the crank reaches e and the piston e1, the exhaust valve is opened, and the gases escape during what remains of the expansion stroke from e1 to f1. The pressure having fallen at f2, to that of the atmosphere, the piston drives out the remaining gases during its travel from f1 to g1, except that retained in the clearance space, and at g1 the inlet valve opens and the cycle is repeated. The exact timing of the valves and the ignition will depend on the engine speed and other conditions and may vary considerably.
