Lesson 4/Learning Event 2
For this system to work, the air must be moved rapidly through the crankcase and out of the road draft tube.
The lower end of the road draft tube is designed to act like a suction pump to get this rapid circulation of air.
Notice that the lower end of the road draft tube is cut at an angle.
Have you ever stuck your hand out of the window of a fast moving car? The wind blowing past your hand
was a pretty powerful force, wasn't it?
If a vehicle with a road draft tube was moving down the road, the wind would be rushing by the end of the
road draft tube at a pretty good clip. With the road draft tube cut at the angle shown, the wind moving by it
would create a suction (known as a partial vacuum) at the end of the road draft tube. This will draw the air
and vapors out of the crankcase.
This system is known as a nonpositive crankcase ventilation system because it will not work unless the vehicle
is moving forward.
A positive crankcase ventilation system operates anytime the engine is running and does not depend on vehicle
movement. Notice that the air enters the crankcase through a breather and goes out through a tube leading to
the intake manifold. Anytime the engine is running there is a low-pressure area in the intake manifold. The
air from the crankcase containing the blowby gases and fuel and water vapors is drawn through the intake
manifold to the combustion chamber. There, the combustible gases are burned and discharged through the
exhaust system.
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