Chapter 4 Principles of Engine Operation 69 Copyright Goodheart-Willcox Co., Inc. the valve face is hardened by the addition of a hard metal, such as Stellite. Some valves are filled with sodium, which helps to transfer additional heat to the top of the valve. Since exhaust valves are subjected to the extreme heat of the exhaust gases, they are often made of higher quality material than intake valves. Valve Seats and Valve Guides When closed, the valves must form a very tight seal to contain the high pressure of compression and combustion. This means the face of the valve must precisely match the profile of the opening in the cylinder head. In some engines, this seating surface is machined directly into the cylinder head, Figure 4-19. In most modern, high-horse- power diesels, however, a separate valve seat is inserted into the head. During engine operation, the valve stem moves up and down inside a valve guide. The valve guide is con- structed of soft, pliable metal, such as bronze, and is press fit into the cylinder head. The guide acts as a cen- tering device to keep the valve from tilting and the valve face perfectly aligned with its seat. A coil spring fits around the top of the valve guide and is usually attached to the valve stem by a retainer and a pair of half collets, Figure 4-20. Pressure applied by the rocker arm forces the valve down, opening the valve port in the cylinder head. When the rocker arm pressure is released, energy stored in the compressed valve spring forces the valve back up, closing the valve port. On some engines, valve rotators are mounted on the valve to turn it with each stroke. The valve rotator tends to help the valve wear more evenly, extending its life. When two valves are used for the same function in a cylinder, a crosshead is added to form a bridge between the valves. This allows the two valves to be opened by a single rocker arm. When a crosshead is used, a guide is installed between the valves to keep the crosshead in the proper position, Figure 4-21. Intake and Exhaust Manifolds Combustion air and exhaust gases are usually chan- neled to and from the combustion chamber through intake and exhaust manifolds, which are attached to the outside of the cylinder head. Exceptions to this arrangement will be covered in later chapters. Intake Manifold The intake manifold (air manifold), Figure 4-22A, dis- tributes clean, filtered air to the intake ports of each cylinder on a four-cycle engine or to the air box on a two-cycle engine. The intake manifold is made of iron or aluminum and Valve Head Valve seat surface machined in head Figure 4-19. Details of an integral valve seat. Some valve seats are installed as a separate piece in the cylinder head. Valve Valve Valve guides Valve seat Valve seat Figure 4-20. Typical intake and exhaust/related valve setup and components. Note the valve guides, which limit valve side-to-side movement in the head. Crosshead Guide Figure 4-21. A crosshead or bridge enables a single rocker arm to control two valves. (Cummins Engine Co., Inc.)