Figure 7-6. Draft Tube System.
movement of the vehicle. Circulation of fresh air will
(3) By strategic location of the breather cap and
occur In the crankcase with the addition of a breather cap
draft tube and the use of baffles, a complete purging of
also located at a point on the crankcase above oil level.
crankcase blowby fumes is ensured.
(1) The negative pressure created at the end of
The draft tube system is obsolete now because it
the draft tube will cause air to be drawn In through the
discharged excessive hydrocarbon emissions directly into
crankcase breather. A wire mesh filter is built into the
the atmosphere. It also did not keep the crankcase as
breather to keep dirt out of the crankcase.
clean as the positive crankcase ventilation system
described in the next subparagraph. This is because it
The draft tube contains a sediment
relied on the movement of the vehicle to activate it. As a
chamber and a wire mesh filter at the point where it
result of this, draft tube-equipped engines were very
attaches to the crankcase. Its purpose is to trap any oil
prone to sludge buildup.
that tries to leave through the draft tube and return it to
b. Positive Crankcase Ventilation (PCV) System
(Fig. 7-7). The positive crankcase ventilation system
utilizes manifold vacuum to purge the