There are many variations in the design of the engine parts, which function as a group to properly direct clean air to intake valves or ports. Regardless of design differences, the function of each kind of part remains basically the same. We cannot cover every type or model of each part of engine air intake systems in this text. Therefore, we will discuss only a few of the common types of each of the principal parts of these systems.

Silencers, Screens, and Cleaners

Air must enter the intake system as quietly and as clean as possible. A diesel engine uses a great quantity of air. Unless a silencer is installed, the air that rushes through the air-cleaning devices will sound like an extremely high-pitched whistle. Consequently, silencers are generally constructed as part of the air-cleaning components.

One type of air-intake silencer assembly is shown in figure 6-4. This type of silencer is used on some General Motors (GM) series 71 engines. The silencer assembly is bolted to the intake side of the blower. (See fig. 6-2.) A perforated steel partition divides the silencer lengthwise into two sections. Air enters the end of the silencer and passes through the inner section into the blower. The noise of the air passes through the silencer where it is reduced by a sound-absorbent, flameproof, felted cotton waste, which fills the outer section of the silencer.

Upon leaving the silencer, the air enters the blower through an air-intake screen. (See fig. 6-4.) The air-intake screen prevents particles of foreign material from entering the engine. Unless it is filtered from the intake air, foreign material might seriously damage the blower assembly and internal engine parts, such as pistons, piston rings, and liners.

The silencer-and-screen assembly just de-scribed is sometimes referred to as a DRY-type cleaner and silencer. Another type of air cleaner and silencer is the VISCOUS type. In both dry and viscous types, intake air is drawn through a fine mesh or screen which filters the air. The mesh of such cleaners may consist of cotton fabric, wire

Figure 6-4.-Air-intake silencer assembly (General Motors series 71).

screening, specially wound copper crimp, or metal wool. The principal difference between cleaners of the dry and viscous types is that the mesh of a viscous-type cleaner is wet, usually, with a medium-weight oil. An air cleaner and silencer assembly of the viscous type is shown in figure 6-5. The filter and silencer of this unit form a cylinder silencing chamber, the ends of which are packed with sound-deadening material. Air enters the silencer through the circumferential surface of the silencing chamber. The filter element fits over the air inlet. The element of the filter consists of a series of oil-wetted wire baffles, which collect any airborne dirt entering the chamber.

Another type of intake-air cleaner and silencer includes an oil bath as part of the assembly. A cross section of an oil bath air cleaner is shown in figure 6-6. This type of air cleaner is referred to as a heavy-duty oil bath air cleaner. In heavy-duty oil bath air cleaners, there are two cleaning elements. One is a removable separator screen; the other is a fixed metal-wool element (metal mesh). Follow along as we explain how the oil bath air cleaner functions.

The air is drawn into the cleaner through an opening in the center of the top. As the air reaches the bottom of this passage, it changes direction and flows up around the outside of the center passage through the metal-wool element. The centrifugal force, caused by the sudden change in direction, traps large particles of dirt in the oil in the bottom of the cleaner. Smaller particles of dirt in the oil picked up by the air are trapped in the metal-wool element. It is important not to overfill the oil reservoir during maintenance, as this will cause oil to be drawn into the engine. Light-duty oil bath air cleaners work in the same way, but because of their size, they are not capable of providing the same volume of air to the engine.

The silencer and cleaner assemblies de-scribed in the preceding paragraphs are representative of the devices that serve to clean

Figure 6-5.-Viscous type of air filter and silencer assembly.

Figure 6-6.-Airflow through a heavy-duty oil bath type of air cleaner.

intake air and to reduce the noise the air makes as it enters the engine. To ensure sufficient cleaning of the intake air, air filters, regardless of type, should be cleaned as specified by the Planned Maintenance System.