The cylinder head is the component that seals the top of the engine block and forms the top of the combustion chamber. In addition, a cylinder head also houses many vital engine components. These components include the valves that allow gases in and out of each cylinder, the spark plug that’s used to ignite the air-and-fuel mixture in each cylin-der, and the camshaft that opens and closes the valves. A cylinder head is attached to the top of an engine block using a series of large bolts.
One cylinder head is used to cover all the cylinders in one row. For ex-ample, since an in-line engine contains only one row of cylinders, it uses only one cylinder head to cover all the cylinders. In a V-type en-gine, the cylinders are arranged in two separate rows. A separate cyl-inder head is needed to cover each row of cylcyl-inders, so a V-type engine contains two cylinder heads. The two cylinder heads on a V-type engine are almost always identical, and one is mounted on each side of the engine block.
The exact design of the cylinder head depends on the make and model of the engine, but all heads have similar characteristics. Cylinder heads are made of either cast iron or, more commonly, cast aluminum.
Aluminum heads are used more frequently on modern engines be-cause of their lighter weight and superior ability to transfer heat. A typical automotive cylinder head is shown in Figure 42A.
As stated, a cylinder head is attached to the very top of an engine block. Since the cylinder head must be able to seal the top of each cyl-inder, the bottom surface of the cylinder head and the top surface of the engine block are machined flat so they’ll fit tightly against each other. The underside of a typical cylinder head is shown in Figure 42B.
In this figure, note that the bottom of the cylinder head contains a re-cessed area for each of the cylinders. This particular cylinder head is from one side of a V-6 engine; therefore, it contains three separate cyl-inder recesses. (Remember that the cylcyl-inders in a V-6 engine are ar-ranged in two separate rows, with three cylinders in each row.) The recessed space for each cylinder head is the combustion chamber where the air-and-fuel mixture is compressed and burned. The exact shape and size of the combustion chamber depends on the make and model of engine. Two common combustion chamber shapes are shown in Figure 43.
As you can see, the combustion chamber also contains the valves that allow the fuel in and the exhaust gases out of the cylinder. The pas-sages that the valves cover and uncover can be seen in Figure 42B. The valves are installed in one of these cylinders, while the valves are re-moved from the other two cylinders.
FIGURE 42—A top view of an automotive en-gine cylinder head is shown in Figure 42A.
The bottom surface of a cylinder head is shown in Figure 42B. Note how the bottom surface of the head is machined smooth to form a tight seal against the top of the engine block.
A close-up view of a similar combustion chamber is shown in
Figure 44. In this figure, you can clearly see the large passages that al-low the fuel to enter and the gases to exit the combustion chamber and cylinder. These passages are called ports. The ports that allow gases to enter the cylinder are called intake ports, and the ports that allow the exhaust gases to leave the cylinder are called exhaust ports. A cylinder will contain an intake port and an exhaust port for each valve in the cylinder.
The cylinder ports are usually arranged in one of two ways. In the first arrangement, the intake ports and exhaust ports are placed on oppo-site sides of the cylinder head (Figure 45A). This type of head is called a cross-flow head. In a cross-flow head, the incoming air-and-fuel mix-ture enters through ports on one side of the head, and the exhaust
PISTON
COMBUSTION CHAMBER QUENCH AREA
SPARK PLUG
VALVE
PISTON COMBUSTION
CHAMBER
SPARK PLUG
VALVE VALVE
(A)
WEDGE COMBUSTION CHAMBER
(B)
HEMISPHERIC COMBUSTION CHAMBER
FIGURE 43—Figure 43A shows a wedge-shaped combustion chamber, while Figure 43B shows a hemi-spheric combustion chamber.
FIGURE 44—A close-up view of a combustion chamber is shown here.
gases exit through ports on the opposite side of the head. In the second type of cylinder head design, both the intake and exhaust ports are lo-cated on the same side of the head (Figure 45B). In this design, the fuel enters the intake ports and the exhaust gases leave the exhaust ports on the same side of the cylinder head.
As stated earlier, the cylinder head must form a tight seal against the engine block to prevent leakage from the combustion chamber area.
You’ve seen how the bottom surface of the cylinder head and the top surface of the engine block are made perfectly flat to assist in forming a tight seal. However, a special gasket called a head gasket must also be placed between these surfaces to form a complete seal. The gasket is usually made of a thick fibrous material, and often has small metal rings molded into it that surround each of the cylinders (Figure 46). A typical cylinder head is attached to the top of the engine block using several large bolts called head bolts. Once the head bolts are tightened, the cylinder head gasket is compressed between the two flat surfaces to form a tight, leak-free seal.