Valve mechanisms may vary considerably in construction and design, even though the function remains the same. The basic types of valve mechanisms are described briefly in Fireman, NAVEDTRA 10520-H. We will go into more detail in the following paragraphs. ACTUATING MECHANISM, as used in this chapter, is that combination of parts that receives power from the drive mechanism and transmits the power to the engine valves. In order for the intake and exhaust valves, fuel injection, and air start to operate, there must be a change in the type of motion. The rotary motion of the camshaft must be changed to a reciprocating motion. The group of parts that, by changing the type of motion, causes the valves of an engine to operate is generally referred to as the VALVE ACTUATING MECHANISM. A valve-actuating mechanism may include the camshaft, cam followers, pushrods, rocker arms, and valve springs. In some engines, the camshaft is so located that pushrods are not needed. In such engines, the cam follower is a part of the rocker arm.

VALVES

The intake and exhaust valves used in internal-combustion engines are of the poppet type. Poppet valves have heads with cone-shaped or beveled edges and beveled seats, which give the valves a self-centering action. (See figs. 4-1 and 4-2.) Exhaust valves are usually made of silicon-chromium steel or steel alloys. Usually, there is a high content of nickel and chromium included in the steel or alloy so that the valves can resist corrosion caused by high-temperature gases. A hard alloy, such as Stellite, is often welded to the seating surface of the valve face and to the tip of the valve stem. The hard alloy increases the wearing qualities of the surfaces, which make contact when the valve closes. Low-alloy steels are generally used for intake valves because these valves are not exposed to the corrosive action of the hot exhaust gases. Consequently, intake valves are capable of longer periods of trouble-free operation.

In some exhaust valves, sodium is used as an agent for cooling the valves. Sodium-filled poppet valves are provided with a chamber that is formed by the hollow stem that extends well up into the valve head. At operating temperatures, the sodium becomes a liquid and splashes up and down inside the hollow valve stem. The sodium is an effective agent that serves to transfer the heat from the hot exhaust valve head through the stem and valve guides and to the engine cooling system. Although sodium-filled exhaust valves are effective, they are not commonly used.

Valve seat inserts (fig. 4-1) are provided in most diesel engine cylinder heads so that valve seat life is extended. Valve seat inserts also have the advantage of being replaceable. Several different types of materials are used in the manufacture of valve seat inserts. Intake valve seats are

Figure 4-1.Valve operating mechanism (Alco 251-C).

manufactured from special alloys of cast iron. Exhaust valve seats are made from stellite and hardened chrome vanadium steel. Valve seat inserts are installed with an interference fit. Replaceable valve guides (fig. 4-1) are provided for most diesel engines. Replaceable valve guides are made of a cast iron alloy that has a superior wearability and is more corrosion resistant than the alloy that is used in the cylinder head. Replaceable valve guides not only provide a guide and bearing for each valve stem but also aid in conducting heat from each valve stem to the water jacket that surrounds the guide.