Remember that the flow of gases in and out of each cylinder is con-trolled by valves mounted in the cylinder head. Each cylinder has its
WEDGE
FIGURE 45—Figure 45A shows a cross-flow cylinder head. In this type of head, the intake and exhaust ports are located on opposite sides of the cylinder head. Note that the intake and exhaust valves are positioned on opposite sides of the spark plug. In the cylinder head shown in Figure 45B, both the intake and exhaust ports are located on the same side of the head. In this head, the intake and exhaust valves are positioned side by side.
own set of valves to control the flow of gases. These valves open and close the ports as the engine operates. During the intake stage, the take valve opens. The air-and-fuel mixture then enters through the in-take port, past the open inin-take valve, and into the cylinder. During the compression stage, both the intake and exhaust valves close com-pletely and prevent any leakage from the cylinder. The valves remain closed as the compressed mixture is burned in the cylinder to produce power. Once the mixture is burned, the exhaust valve opens and al-lows the remaining burned gases to escape out through the exhaust port.
The valves used in automotive engines are called poppet valves. A pop-pet valve, in simple terms, is a round metal disk with a long shaft.
Figure 47 shows the parts of a typical intake and exhaust valve. The long shaft of the valve is called the valve stem. The large-diameter round end of the valve is called the valve head. The surface where the valve head fits against the valve seat is machined at an angle to help form a seal. This angled sealing surface is called the valve face.
The small area between the valve head and valve face is called the valve margin. The valve margin is important to the operation of the valve. If the valve margin is too thin, the intense heat inside the engine will simply melt the margin away and prevent the valve from sealing.
A melted or burned valve margin is commonly called a burnt valve.
The final parts of the valve shown in Figure 47 are the retainer lock grooves. These grooves are cut into the end of the long valve stem. The
FIGURE 46—A typical automotive cylinder head gasket is shown here. (Photo courtesy of Mr. Gasket Co.)
locking parts that keep the valve from falling out of the valve guide are attached to these grooves.
Both the intake and exhaust valves are similar in appearance—the only real difference is their size. In most engines, the intake valve head is larger than the exhaust valve head. There are several reasons for this, but the main reason is heat. During the power stage, the temperature inside the combustion chamber gets very hot from the burning of the air-and-fuel mixture. Since the valves are in the combustion chamber area, they’re directly exposed to this heat. Once the burning is com-plete, the hot gases remaining from the burning process exit the cylin-der by passing the open exhaust valve. As these hot exhaust gases pass over the valve, the exhaust valve's temperature rises even more.
In contrast, the temperature of the intake valve isn’t increased during combustion. In fact, the intake valve is actually cooled by the fresh air-and-fuel mixture that’s brought into the cylinder during the intake stage. The temperature of the incoming mixture is relatively cool com-pared to the valve, so the mixture cools the valve as it passes through.
Since the intake valve stays much cooler than the exhaust valve, the in-take valve can be larger in size without the concern of melting the valve margin.
Thus, since the exhaust valve is exposed to higher temperatures in the engine, it’s usually made of a heavy material and is smaller than the in-take valve. This helps to prevent the edges of the exhaust valve from burning away. Unlike the intake valve, which is cooled by the incoming fuel mixture, the exhaust valve can cool itself only by transferring heat to the cylinder head. This metal-to-metal heat transfer occurs through the valve guide, or through the valve seat whenever the valve is closed.
Now, let’s examine how a valve is installed in an engine. A typical valve installation is shown in Figure 48. In the figure, note that when a valve is closed, it seals against a specially machined area in the
FIGURE 47—The parts of an intake and exhaust valve are shown here.
Note that the intake valve is larger than the exhaust valve.
cylinder head called the valve seat. The valve seats are simply round, machined holes located at the edge of the cylinder head ports. As you can see in this illustration, the valve is in the open position, allowing gases to flow through the port and into the engine. When the valve is in its closed position, it fits tightly against the valve seat, forming a tight seal that will prevent any leakage out of the cylinder. The valve and valve seat are machined to such precise specifications that they seal with only metal-to-metal contact. No additional gasket or seal is used between them.
As we mentioned earlier, a valve has a long metal stem. In Figure 48, you can see how the long valve stem fits into a long, round tube in the cylinder head. This long tube is called a valve guide. The valve guide holds the valve in place, while still allowing it to open and close. The valve guide is slightly larger in diameter than the valve stem, which allows the stem to slide easily in and out of the guide. However, the guide is narrow enough to prevent the valve stem from moving side to side. In some engines, the valve guide is machined directly into the cylinder head, while in other engines, the guide is a separate part that’s press-fit tightly into the cylinder head. Either way, the valve guide per-forms the same function.