r, the connecting-rod; k, the crankpin; q, the crank-shaft; and a, the crank. In the indicator diagram, the ordinates or vertical distances represent pressures, and the abscissas or horizontal distances denote the distance the piston has proceeded on its stroke. The pressures are measured from the dotted line OV, which represents the pressure of the atmosphere. The line u is the vacuum line, the full line OvV is the suction line, and the line Vf is the compression line. At f, the charge is ignited, and fg denotes compression and explosion combined; gh, the vertical explosion line; hj, the expansion line; jV, combined expansion and exhaust; and VwO is the exhaust line. The pressures represented by the two lines v and w are slightly exaggerated, in order that the lines may be distinguished from the atmospheric line oV, which they follow very closely.
32. In Fig. 8 is illustrated the suction stroke, the crank-shaft turning in the direction of the arrow, and the piston having moved from the dotted line n to the position shown. The space between the end of the cylinder, when at the line n, and the cylinder head is called the clearance space or the combustion chamber. It should be noted that the inlet valve s is open and the mixed air and gas is being drawn into the cylinder, as shown by the arrows. The pressure within the cylinder drops slightly below the atmosphere, as shown by the line v. The valve s remains open until the piston gets to the right-hand end of its stroke. The numbers at the left of the diagram represent the pressures, and those at the bottom the volumes, corresponding to the cross-lines opposite which they are written.
33. When the piston starts on its return stroke, Fig. 9, the valve s is closed and the mixture is trapped within the cylinder and compressed. The rise of pressure during compression is shown in the indicator diagram by the line Vf. When the compression has proceeded to f, a spark is produced by the igniter i and combustion begins. The rise of pressure from f to g is therefore due both to the compression and to the combustion of the gas. The flame spreads
