Diesel Engine FundamentalsDOE-HDBK-1018/1-93DIESEL ENGINESAirIntakeSystemBecause a diesel engine requires close tolerances to achieve its compression ratio, andbecause most diesel engines are either turbocharged or supercharged, the air entering theengine must be clean, free of debris, and as cool as possible. Turbocharging andsupercharging are discussed in more detail later in this chapter. Also, to improve aturbocharged or supercharged engine's efficiency, the compressed air must be cooled afterbeing compressed. The air intake system is designed to perform these tasks. Air intake systems vary greatlyFigure 14 Oil Bath Air Filterfrom vendor to vendor but areusually one of two types, wet ordry. In a wet filter intake system,as shown in Figure 14, the air issucked or bubbled through ahousing that holds a bath of oilsuch that the dirt in the air isremoved by the oil in the filter.The air then flows through ascreen-type material to ensure anyentrained oil is removed from theair. In a dry filter system, paper,cloth, or a metal screen material isused to catch and trap dirt beforeit enters the engine (similar to thetype used in automobile engines).In addition to cleaning the air, theintake system is usually designedto intake fresh air from as faraway from the engine aspracticable, usually just outside ofthe engine's building or enclosure.This provides the engine with asupply of air that has not beenheated by the engine's own wasteheat. The reason for ensuring that an engine's air supply is as cool as possible is that cool airis more dense than hot air. This means that, per unit volume, cool air has more oxygenthan hot air. Thus, cool air provides more oxygen per cylinder charge than less dense,hot air. More oxygen means a more efficient fuel burn and more power. Rev. 0ME-01Page 15
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