In all of the diagrams the crank is supposed to turn in a right handed direction as indicated by the arrow, the piston moving in the direction shown by the arrow under the piston head. The valves are operated by cams on an intermediate shaft known as the “cam shaft.” As the valves go through their series of movements in two revolutions of the crank shaft, and as the cam shaft must perform all of these operations in one revolution, it is evident that the cam shaft must run at exactly one-half the crank-shaft speed. This change of speed is accomplished by means of gearing between the cam shaft and crank-shaft from which the cam shaft is driven. In some engines, notably the Diesel engine, pure air is drawn into the cylinder on stroke No. 1 instead of the entire mixture. Fuel is supplied in this type immediately after the end of the compression stroke. While an electric spark is shown as the igniting medium in the diagrams, the ignition is sometimes performed by a hot tube, or simply by the heat of the compression as in the Diesel engine. In the sliding sleeve type of four stroke cycle motor, the poppet or lifting type of valve as shown in Fig. 4, is replaced by a peculiar type of slide valve similar in action to the slide valves used on steam engines, except that it is cylindrical in form and entirely surrounds the piston. While there is a change in the form of the valve, and in a number of small details, the gases are drawn into the cylinder, compressed, ignited, and released in exactly the same way and in the same rotation, as in the poppet valve engine just described. A description of the Knight engine which is the most prominent example of the slide sleeve motor will be found in a succeeding chapter. Since the success of the slide valve type has been acknowledged by many prominent automobile manufacturers, there have been several similar types placed on the market, some with two sleeves and some with one, but in all cases the designers have had but two points in view, that is quiet running and free passages. (26) Two Stroke Cycle Engine. Two stroke cycle engines perform the five events of aspiration (suction), compression, ignition, expansion and release in two strokes or one revolution. Providing that these events are performed as efficiently as in the four stroke cycle engine, it is evident that with equal cylinder capacity, the two stroke cycle engine would have twice the output of a four stroke cycle since it gives twice the number of impulses per revolution. Unfortunately it is impossible to attain twice the output of the four stroke cycle type with the small two stroke engines built at the present time because of their imperfect scavenging and poor fuel economy. In the larger two stroke engines, the pumps and blowers used for scavenging the cylinders consume a considerable percentage of the output. [Illustration: Fig. 5. Diagram of Two Port—Two Stroke Cycle Engine, Showing the Events in the Crank-Case and Cylinder. ]