This type of valve, while not at all new, has only within the past few years come into considerable prominence, chiefly as a result of the truly remarkable performances of the Knight motor, which is equipped with the most advanced examples of this type of valve.
Contrary to past opinion, it has been conclusively demonstrated that sleeve valves do not in any perceptible degree increase the tendency of a motor to overheat, nor do they wear at any very measurable rate. They afford, moreover, in the best constructions, a much higher thermal and mechanical efficiency than it is possible to secure from the average poppet-valve motor, this improvement being due to the better-shaped combustion chamber that can be used, and the greater areas of valve opening, which facilitate the ingress and egress of the charges.
Another advantage in favor of the sleeve valve is that its timing is permanent and unchangeable, and does not alter materially with wear. Not the least of the merits of the sleeve valve is found in the fact that it lends itself to positive operation by eccentric mechanisms, which are in every way greatly superior to the non-positive cam mechanisms universally used to actuate poppet valves.
A very good example of this latest type of Knight motor is illustrated in Fig. 9, showing the intake side of the Moline-Knight four-cylinder motor.
Sliding valves of other than the sleeve type, embracing a considerable variety of piston valves and valves similar to those employed in steam engines, have not found as much favor with designers of automobile engines as have other types herein referred to.
One exception is the successful use of a "split-ring" valve, sliding up and down in the cylinder head just above the piston, which has found successful application in a few motors recently built by the Renault Company, of France.
A rotating valve of characteristic type is that employed in the single-cylinder engine illustrated in Fig. 10, which is an experimental motor designed by Anzani, famous as the designer of several European automobile motors, and of the aviation motor with which Bleriot effected the first flight across the English Channel.
Fig. 11. Sections of Darracq Rotating Valve Motor, Showing Intake Position (left) end Exhaust Position (right).
Other rotating valves that have proved successful are the Darracq valve, illustrated in Figs. 11 and 12, and various rotating inlet valves used on the crankcases of two-cycle motors.
The Darracq rotating valve is a particularly clever example of sound designing, and exhaustive tests have proved it thoroughly successful and reliable.
Much of its merit undoubtedly inheres in the fact that the port through which it communicates with the cylinder is closed by the piston at the top of the stroke, so that at the moment of explosion the valve is shielded from the highest temperatures that occur within the cylinder.
An American motor of somewhat similar general form has a pair of these valves on opposite sides of the cylinder head, driven, however, by silent chains. The only difference in the action from that described and illustrated is the simplification and reduction of sizes made possible by having the exhaust on one side and the inlet on the other.
Fig. 12. Complete Darracq Rotating Valve Motor This Photograph Protected by International Copyrighl.
For the actuation of the valve mechanism of any four-cycle motor, it is necessary to have a shaft (or in the case of rotary valves, to run the valve itself as a shaft), turning at one-half the speed of the crankshaft through a two-to-one gear ratio.
Ordinarily the half-time shaft is the camshaft, but in motors of the Knight type it is, of course, an eccentric shaft. Camshafts particularly call for good workmanship and high-grade materials, as well as sound design, since the constant pounding of the valve stems or push rods on the cams is a prolific source of trouble, if anything but the soundest of sound construction be employed.
The most important recent innovation in this detail of automobile mechanism is the driving of half-time shafts by silent chains in place of the long-used gearing, of spur and helical type. By this improvement the noise of the gears is eliminated.
A typical silent chain running over a pair of gears may be seen in Fig. 13. These, however, present very broad-faced gears, while the usual timing gears would have a narrow face. In the use and action of the silent chain, this makes little or no difference. In the Cadillac motor, shown in Fig. 2, a pair of these is used, one driving the camshaft from the crankshaft while the other drives the auxiliary shaft from the camshaft. In the American form of Knight sliding-sleeve-valve motors shown in Fig. 9, a pair of silent chains is used for the eccentric shaft on one side and the electric generator on the other. These are driven from a pair of sprockets set side by side on an extension of the crankshaft.
A point that should be brought out in connection with silent-chain camshaft driving is that the use of the chain allows the shafts to be placed anywhere desired, and thus, to a certain extent, frees the designer from the former restriction of a two-to-one reduction ratio in the gears, which rather fixed the size, and consequently the position of the gears. This had an influence also upon cylinder design, as the center of the camshaft fixed the center of all the valves - that is their distance from the center line of the motor.
Probably the most important thing about a four-cycle gasoline engine is the valve, or more correctly, are the valves, for the usual number is two per cylinder. The opening and closing of these control the functions of the engine, for if the valve does not open and allow a charge of gas to enter, how can the piston compress, and the ignition system fire, a charge? Similarly, if the exhaust valve is not opened and the burned gases allowed to escape, they will mingle with and dilute the fresh, incoming charge, possibly to the extent of making the latter into a non-combustible gas.
Fig. 13. Silent Chun End Sprocket).