Valves and Valve Mechanisms

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Valves and Valve Mechanisms

Most engines have two valves for each cylinder, one intake and one exhaust valve. Since each of these valves operates at different times, separate operating mechanisms must be provided for each valve. Valves are normally held closed by heavy springs and by compression in the combustion chamber. The purpose of the valve-actuating mechanism is to overcome the spring pressure and open the valves at the proper time. The valve-actuating mechanism includes the engine camshaft, camshaft followers (tappets), pushrods, and rocker arms.

CAMSHAFT.The camshaft (fig. 12-23) is enclosed in the engine block. It has eccentric lobes (cams) ground on it for each valve in the engine. As the

Figure 12-24.-L-head valve operating mechanism.

camshaft rotates, the cam lobe moves up under the valve tappet, exerting an upward thrust through the tappet against the valve stem or a pushrod. This thrust over-comes the valve spring pressure as well as the gas pressure in the cylinder, causing the valve to open. When the lobe moves from under the tappet, the valve spring pressure reseats the valve.

On L-, F-, or I-head engines, the camshaft is usually located to one side and above the crankshaft; in V-type engines, it is usually located directly above the crankshaft. On the overhead camshaft engine, such as the Murphy diesel, the camshaft is located above the cylinder head.

The camshaft of a four-stroke cycle engine turns at one-half engine speed. It is driven off the crankshaft through timing gears or a timing chain. In the two-stroke cycle engine, the camshaft must turn at the same speed as the crankshaft so that each valve may open and close once in each revolution of the engine.

In most cases the camshaft will do more than operate the valve mechanism. It may have extra cams or gears that operate fuel pumps, fuel injectors, the ignition distributor, or the lubrication pump.

Camshafts are supported in the engine block by journals in bearings. Camshaft bearing journals are the hugest machined surfaces on the shaft. The bearings are usually made of bronze and are bushings rather than split bearings. The bushings are lubricated by oil circulating through drilled passages from the crankcase. The stresses on the camshaft are small; therefore, the bushings are not adjustable and require little attention. The camshaft bushings are replaced only when the engine requires a complete overhaul.

FOLLOWERS. Camshaft followers are the parts of the valve-actuating mechanism (figs. 12-24 and 12-25) that contact the camshaft. You will probably hear them called valve tappets or vale lifters. In the L-head engine, the followers directly contact the end of the valve stem and have an adjusting device in them. In the overhead valve engine, the followers contact the pushrod that operates the rocker arm. The end of the rocker arm opposite the pushrod contacts the valve stem. The valve adjusting device, in this case, is in the rocker arm.

Many engines have self-adjusting, hydraulic valve lifters that always operate at zero clearance.

Figure 12-25.Valve operating mechanism for an overhead valve engine.

Figure 12-26.-Operation of a hydraulic valve lifter.

Figure 12-26 shows the operation of one type of hydraulic valve tappet mechanism. Oil under pressure is forced into the tappet when the valve is closed. This pressure extends the plunger in the tappet so that all valve clearance, or lash, is eliminated. When the cam lobe moves around under the tappet and starts to raise it, you hear no tappet noise. The movement of the tappet forces the oil upward in the lower chamber of the tappet. This action closes the ball check valve so that oil cannot escape. Then the tappet acts as though it were a simple, one-piece tappet and the valve is opened. When the lobe moves out from under the tappet and the valve closes, the pressure in the lower chamber of the tappet is relieved. Any slight loss of oil from the lower chamber is replaced by the oil pressure from the engine lubricating system. This oil pressure causes the plunger to move up snugly against the push rod so that any clearance is eliminated.