The liners or bores of an internal-com-bustion engine must be sealed tightly to form the combustion chambers. In Navy engines, except for engines of the opposed-piston type, the space at the combustion end of a cylinder is formed and sealed by a cylinder head that is a separate unit from the block or liner.

A number of engine parts that are essential to engine operation may be found in or attached to the cylinder head. The cylinder head for a 4-stroke cycle engine will house intake and exhaust valves, valve guides, and valve seats; whereas for a 2-stroke cycle engine, the head will carry many of the same parts with the exception of the intake valves. Rocker arm assemblies are frequently attached to the cylinder head. The fuel injection valve is almost always in the cylinder head or heads of a diesel engine, while the spark plugs are always in the cylinder head of gasoline engines. Cylinder heads of a diesel engine may also be fitted with air starting valves, indicator cocks, and safety valves.

The design and material of a cylinder head must be such that it can withstand the rapid changes of temperature and pressure that take place in the combustion space and the mechanical stress that results from the head being bolted securely to the block. Cylinder heads are made of heat-resistant alloy cast iron or aluminum alloy. The number of cylinder heads found on diesel engines varies considerably. Small engines of the in-line cylinder arrangement use one head for all cylinders. A single head may cover all the cylinders in each bank in some V-type engines. Large diesel engines generally have one cylinder head for each cylinder, although some engines use one head for each pair of cylinders.

A cylinder head of the type used to seal all cylinders of one bank of a V-type block is shown in figure 3-15. The cylinder head, over each bank

Figure 3-15.A multiple cylinder head for a V-type engine..

of cylinders, is a one-piece casting which can be removed from the engine as an assembly containing such moving parts as the cam followers and guides, push rods, rocker arms, exhaust valves, and fuel injectors. The cylinder head shown in views A, B, and C of figure 3-16 is used when one cylinder head is required for each cylinder. The cylinder head is made of high-strength, cast-iron alloy with specially designed cast passages for water and exhaust gases. Drilled holes (view C) at the bottom of the cylinder head match the water discharge holes in the liner. Other passages in the cylinder head line up with mating elbows in the cylinder block to conduct exhaust gases through the water manifold to the exhaust manifold. Another opening located in the center of the cylinder head (view A) is where the fuel injector is installed.

Another cylinder head of the type used to seal one cylinder of a block is shown in figure 3-17. This cylinder head is different in construction compared to the heads we have discussed. The cylinder heads shown in figures 3-15 and 3-16 must be removed from the block before the exhaust valves can be serviced. The cylinder head in figure 3-17, however, features a caged design that allows for replacement of the exhaust valves and seats in minimal time without removal of the secured cylinder head. When the intake valves re-quire servicing, the cylinder head must be removed from the block. Additional information on valves is contained in chapter 4 of this manual.