VALVE OPERATION
The methods of valve operation commonly used vary according to the type of cylinder construction employed. In all cases the valves are lifted from their seats by cam-actuated mechanism. Various forms of valve-lifting cams are shown at [Fig. 101]. As will be seen, a cam consists of a circle to which a raised, approximately triangular member has been added at one point. When the cam follower rides on the circle, as shown at [Fig. 102], there is no difference in height between the cam center and its periphery and there is no movement of the plunger. As soon as the raised portion of the cam strikes the plunger it will lift it, and this reciprocating movement is transmitted to the valve stem by suitable mechanical connections.
Fig. 101.—Forms of Valve-Lifting Cams Generally Employed. A—Cam Profile for Long Dwell and Quick Lift. B—Typical Inlet Cam Used with Mushroom Type Follower. C—Average Form of Cam. D—Designed to Give Quick Lift and Gradual Closing.
The cam forms outlined at [Fig. 101] are those commonly used. That at A is used on engines where it is desired to obtain a quick lift and to keep the valve fully opened as long as possible. It is a noisy form, however, and is not very widely employed. That at B is utilized more often as an inlet cam while the profile shown at C is generally depended on to operate exhaust valves. The cam shown at D is a composite form which has some of the features of the other three types. It will give the quick opening of form A, the gradual closing of form B, and the time of maximum valve opening provided by cam profile C.
Fig. 102.—Showing Principal Types of Cam Followers which Have Received General Application.
The various types of valve plungers used are shown at [Fig. 102]. That shown at A is the simplest form, consisting of a simple cylindrical member having a rounded end which follows the cam profile. These are sometimes made of square stock or kept from rotating by means of a key or pin. A line contact is possible when the plunger is kept from turning, whereas but a single point bearing is obtained when the plunger is cylindrical and free to revolve. The plunger shown at A will follow only cam profiles which have gradual lifts. The plunger shown at B is left free to revolve in the guide bushing and is provided with a flat mushroom head which serves as a cam follower. The type shown at C carries a roller at its lower end and may follow very irregular cam profiles if abrupt lifts are desired. While forms A and B are the simplest, that outlined at C in its various forms is more widely used. Compound plungers are used on the Curtiss OX-2 motors, one inside the other. The small or inner one works on a cam of conventional design, the outer plunger follows a profile having a flat spot to permit of a pull rod action instead of a push rod action. All the methods in which levers are used to operate valves are more or less noisy because clearance must be left between the valve stem and the stop of the plunger. The space must be taken up before the valve will leave its seat, and when the engine is operated at high speeds the forcible contact between the plunger and valve stem produces a rattling sound until the valves become heated and expand and the stems lengthen out. Clearance must be left between the valve stems and actuating means. This clearance is clearly shown in [Fig. 103] and should be .020′′ (twenty thousandths) when engine is cold. The amount of clearance allowed depends entirely upon the design of the engine and length of valve stem. On the Curtiss OX-2 engines the clearance is but .010′′ (ten thousandths) because the valve stems are shorter. Too little clearance will result in loss of power or misfiring when engine is hot. Too much clearance will not allow the valve to open its full amount and will disturb the timing.