the engine being of the double-acting type. Because of the extreme wastefulness of this engine, which was of the two-cycle type, a French scientist, Beau de Rochas, in 1870, proposed a new cycle of operations. This cycle was adopted and put into practical use by Herr Otto, a German, who built his first compression engine in 1876. From this latter circumstance, the cycle came to be known as the Otto cycle, although the honor of its invention belongs unquestionably to Beau de Rochas.
30. Application of Four-Cycle Principle. — In the four-cycle engine, the first outward stroke is the suction stroke, the gas being driven into the cylinder by the pressure of the atmosphere or other pressure, because of the partial vacuum produced by the movement of the piston. This stroke fills the cylinder with a mixture of fuel and air at very nearly the pressure of the atmosphere. On the return stroke of the piston, all the openings leading from the cylinder are closed and the mixture is compressed. As the piston nears the end of this second stroke, which is known as the compression stroke, the igniter, or device, by means of which the charge is fired, is operated in time to produce full ignition of the mixture at the end of the stroke. The pressure rises to three or four times that due to compression, and drives the piston forwards on its next outward stroke, which is known as the expansion, or power, stroke. Just before, or as this stroke is completed, the exhaust valve is opened, permitting the burned gas and uncombined air to escape to the atmosphere, and during the following inward stroke practically all of this waste material is expelled. This last stroke is known as the exhaust stroke.
31. Graphic Representation of Four-Stroke Cycle. The four strokes of the engine cycle are shown in Figs. 8, 9, 10, and 11; above each figure is given an indicator diagram in which the line produced during the stroke shown in the figure is a full line, while the remainder of the diagram is shown in dotted lines. In all the figures, p denotes the piston; c, the cylinder; e, the exhaust valve; s, the inlet valve; z the igniter,
