Fig. 10.—Automatic Inlet Valve in Cage.
The valve openings, or seats, are circular, and are usually made slightly funnel shaped, the disks that cover them being slightly conical to fit. The large end of the funnel is toward the combustion space, so that when the disk is lifted from its seat it moves inward. Valves are held against their seats by coil springs that surround the valve stems, which are rods extending from the center of the disks, and there are two methods by which they are opened. In an automatic valve, the spring that holds the disk against its seat is weak, and the higher pressure outside of the cylinder during the suction stroke forces the disk away from its seat against the pressure of the spring. The valve remains open until the pressure in the combustion space is about equal to that outside, when the spring draws the disk back to its seat, to which it is held as long as the pressure inside is higher than that of the atmosphere. This arrangement is only possible for inlet valves, and is largely used, but exhaust valves, and often inlet valves as well, are mechanically operated; that is, they are opened and held open by a mechanism driven by the crank shaft, in the form of a cam. A cam can best be described as a “wheel with a hump on it,” or, in other words, it is a piece of metal mounted on a shaft, cylindrical in form except for one portion, which projects farther from the shaft than the rest. The cam revolves with the shaft, and the projection, called the nose, will displace anything resting against it. The illustration shows a cam in three positions of its revolution, with the end of a valve stem resting against it—the roller being attached to the stem to reduce the friction. The valve stem is held in guides, so that the only movement it may have is up and down; when the cam revolves, the nose lifts the stem and opens the valve, holds it open as long as the flat end of the cam is under the stem, and when the nose passes from under, the valve is drawn to its seat by the spring.
Fig. 11.—Cam Action.
The moment at which an automatic valve opens is governed partly by the tension of its spring; if it is too strong, greater pressure will be required to open it, and it will close sooner than if the tension is light. Accurate adjustment of this spring is necessary in order that the charge may enter the combustion space without delay, and continue to enter as long as possible. The opening and closing of mechanically operated valves depend on the shape of the cam, and not being affected by the more or less uncertain action of a spring, they are more positive in action.
The cam shaft on which the cam is mounted is driven by the crank shaft, but as the valve opens but once during two revolutions, the cam shaft revolves at half speed, making one revolution while the crank shaft makes two. This is done by means of gears.
If two gears running together, or in mesh, have the same number of teeth, they will make the same number of revolutions, but if one has twice as many teeth as the other, the smaller will revolve twice while the larger revolves once. As the cam shaft must revolve but once while the crank shaft revolves twice, its gear must have twice the number of teeth as the gear on the crank shaft. The cam shaft is also called the secondary, or half-time shaft, and the gears that drive it the two-to-one gears.
In some designs of engines, the nose of the cam bears directly against the valve stem, but it is more usual to place a valve-lifter rod, or push rod, between them, the cam acting on the rod and lifting it, and that in turn lifting the valve stem. When the nose of the cam is not acting on the stem or rod, there must be a small space between them, for if the stem or rod rests firmly against the cam at all times, the valve disk might be prevented from seating firmly. The space is left between the stem and lifter rod, the spring acting only on the stem.
Fig. 12.—Four Arrangements of Valves.