Diesel Engine Fundamentals
DOE-HDBK-1018/1-93
DIESEL ENGINES
Air Intake System
Because a diesel engine requires close tolerances to achieve its compression ratio, and
because most diesel engines are either turbocharged or supercharged, the air entering the
engine must be clean, free of debris, and as cool as possible. Turbocharging and
supercharging are discussed in more detail later in this chapter. Also, to improve a
turbocharged or supercharged engine's efficiency, the compressed air must be cooled after
being compressed. The air intake system is designed to perform these tasks.
Air intake systems vary greatly
Figure 14 Oil Bath Air Filter
from vendor to vendor but are
usually one of two types, wet or
dry. In a wet filter intake system,
as shown in Figure 14, the air is
sucked or bubbled through a
housing that holds a bath of oil
such that the dirt in the air is
removed by the oil in the filter.
The air then flows through a
screen-type material to ensure any
entrained oil is removed from the
air. In a dry filter system, paper,
cloth, or a metal screen material is
used to catch and trap dirt before
it enters the engine (similar to the
type used in automobile engines).
In addition to cleaning the air, the
intake system is usually designed
to intake fresh air from as far
away
from
the
engine
as
practicable, usually just outside of
the engine's building or enclosure.
This provides the engine with a
supply of air that has not been
heated by the engine's own waste
heat.
The reason for ensuring that an engine's air supply is as cool as possible is that cool air
is more dense than hot air. This means that, per unit volume, cool air has more oxygen
than 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.
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