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.
Fig. 5. Diagram of Two Port—Two Stroke Cycle Engine, Showing the Events in the Crank-Case and Cylinder.
A general classification of the two stroke cycle engine is not so simple a matter as that of the four stroke because of the differences in construction of large and small sizes. This difference between the large stationary engine and the small type commonly used on boats is due to the efforts of the builders of the large engine to obtain great fuel economy, while the chief endeavors of the builders of small engines is to build a simple and reliable engine for the use of inexperienced persons. While the smaller type of two stroke engine (less than 25 horse-power) has not been used in stationary practice to any extent, owing to the defects just named, or on automobiles, it has been widely used on motor boats, a service for which it is peculiarly adapted. Its extended use on boats is due to the fact that in such service it runs at practically a constant speed and works against a steady load, the conditions that are most favorable to the type. With automobiles where the motor speed is constantly varying, as well as the load, this type of motor is not flexible enough to meet the continually varying conditions.
The small two stroke motors are divided into two principal classes, the two port and three port type, depending on the method by which the charge is transferred to the cylinder. No valves are used in the cylinders of either type for the admission or release of the gases. As the two strokes of the cycle are the compression stroke and working stroke, it is evident that the charge must be introduced into the cylinder by means other than by the suction of the piston and at a time when there is no pressure in the cylinder. This is accomplished by a preliminary compression of the charge in the crank case which places the mixture under sufficient pressure to force it into the cylinder at the end of the working stroke and at the same time to displace the burnt gases left from the previous explosion. It should be noted that the incoming mixture is a substitute for both the suction and scavenging strokes of the four stroke cycle engine.