o, the inlet, or transfer, port; b, the passage leading from the crank-chamber to the cylinder; s, the inlet valve; d, a deflector on the end of the piston; and i, the part of the igniting device at which the spark is produced. The diagram of pressures in the cylinder are shown above the engine and to the left, while the diagram for the pressures in the crank-case are shown to the right.
The difference between the diagram of this engine and that of the four-cycle engine should be carefully noted. In Fig. 18, the piston is moving toward the cylinder head, compressing the mixture of gas and air, while at the same time it is drawing a new charge into the crank-case through the valve s. That portion of the diagrams given during this stroke is shown by full lines. In reality, the first part of the cycle must always be the suction into the crank-case before any mixture is taken into the cylinder. The line Vfgh is identical with the compression and explosion line of the four-stroke cycle, as shown in Fig. 9, and covers the same series of operations; namely, compression to f, where ignition takes place, increase of the rate at which the pressure rises from f to g, and the vertical explosion line gh. While the piston is compressing the charge in the cylinder, the crank-case is drawing more fuel through the valve s, the pressure in the crank-case falling below the atmosphere, as shown by the line v below O' V'. It should be noted that the diagrams for the pressures in the crank-case have a different scale of pressures from the scale of the diagrams for the pressures in the cylinder.
52. The next stroke of the piston is shown in Figs. 19 and 20. In Fig. 19, the piston is moving away from the head end, making the expansion stroke for the cylinder and the compression stroke for the crank-case, the inlet valve s being closed. This figure represents that portion of the stroke before the exhaust port e is uncovered, giving the portion of the indicator diagram from h to j for the cylinder and from O' to c' for the crank-case.
53. In Fig. 20, the piston is very near the end of its stroke, both the inlet and the exhaust ports o and e are open,
