The amount of air that enters the cylinders from the air box or manifold is controlled by the opening and closing of the cylinder valves or ports. Whether air is admitted to the cylinders of engines through ports or by valves depends upon the type of engine. As we stated earlier, in 4-stroke cycle engines, the amount of air that enters the cylinders is controlled by valves, while ports are used for this purpose in 2-stroke cycle engines. Ports control the discharge of exhaust gases from some 2-stroke cycle engines, and valves perform the same function in all 4-stroke and in many 2-stroke cycle engines.

We have made frequent reference in this chapter to ports and have illustrated their location in the cylinder liner. (See figs. 6-1, 6-2, and 6-8.) INTAKE PORTS are usually located in such a way that air enters the cylinder in a whirling motion. The turbulence created helps to increase the amount of intake air that is reached by the injected fuel particles. Thus, the power output of the engine is increased. Intake ports as well as EXHAUST PORTS are opened and closed by the piston as it moves back and forth in the cylinder. The valves of an engine are opened and closed at the proper point in the cycle of operation by the valve mechanisms.

Cylinder Test Valves and Safety or Relief Valves

In some large engines, each cylinder is equipped with valves that serve different purposes from those of the intake and exhaust valves. Instead of admitting air to the cylinder or permitting the exhaust gases to escape, these valves may be used for testing or safety purposes. Even though they are not a part of the air system of the engine, these valves are definitely related to the air system since they serve to test or relieve pressure which may develop with the combustion space.

A TEST VALVE is used (1) to vent the cylinder of any accumulated water or oil before an engine is started; (2) to relieve cylinder pressure when the engine is being turned by hand; or (3) to test compression and firing pressures. Test valves are hand operated. (See fig. 6-13.) Manufacturers use the terms safety valves and relief valves to identify valves installed in cylinder heads or liners that serve to relieve excessive pressure that may develop when the engine is operating. These valves are of the spring-loaded poppet type. Whether they are called safety valves or relief valves, these valves are designed to open when the cylinder pressure exceeds a safe operating limit.

A sectional view of the safety valve and test valve arrangement is shown in figure 6-13. Note the passage and adapter for a cylinder pressure indicator. Most test and relief valve arrangements have an adapter for the cylinder pressure measuring instrument.

The valves shown in figure 6-13 are fitted in passages within the cylinder head. In some engines, however, the relief valve is attached separately to the exterior of the head or liner. For example, the cylinder relief valve of a Fairbanks-Morse 38D8 1/8 is screwed into an adapter which also has a tapped opening for an indicator valve.

Figure 6-13.-Cylinder test and safety valves.

The adapter is attached to the cylinder liner adapter sleeve.