POSITIVE CRANKCASE VENTILATION
crankcase ventilation system uses manifold vacuum to purge the crankcase blow-by fumes.
The fumes are then aspirated back into the engine where they are reburned.
A hose is
tapped into the crankcase at a point that is well above the engine oil level. The other
end of the hose is tapped into the intake manifold or the base of the carburetor.
If the hose is tapped into the
carburetor base, it will be in a location that is between the throttle valves and the
intake manifold so that it will receive manifold vacuum.
breather is installed on the crankcase in a location that is well above the level of the
engine oil. The inlet breather also is located strategically to ensure complete purging of
the crankcase fresh air. The areas of the crankcase where the vacuum hose and inlet
breather are tapped have baffles to keep motor oil from leaving the crankcase.
A flow control
valve is installed in the line that connects the crankcase to the manifold. It is called a
positive crankcase ventilation (PCV) valve (fig. 4-52) and serves to
avoid the air-fuel mixture by doing the following:
periods of large throttle opening will be accompanied by heavy engine loads. Crankcase
blow-by will be at its maximum during heavy engine loads. The PCV valve will react to the
small amount of manifold vacuum that also is present during heavy engine loading by
opening fully through the force of its control valve spring. In this way, the system
provides maximum effectiveness during maximum blow-by periods.
period of small throttle opening will be accompanied by small engine loads, high manifold
vacuum, and a minimum amount of crankcase blow-by. During these periods, the high manifold
vacuum will pull the PCV valve to its position of minimum opening. This is important to
prevent an excessively lean air-fuel mixture.
the event of engine backfire (flame traveling back through the intake manifold), the
reverse pressure will push the rear shoulder of the control valve against the valve body.
This will seal the crankcase from the backfire which could otherwise cause an explosion.
The positive crankcase ventilation system can be either the open or closed type (fig. 4-52).
open type has an inlet breather that is open to the atmosphere. When this system is used,
it is possible for a portion of the crankcase blow-by to escape through the breather
whenever the engine is under a sustained heavy load.
closed type has a sealed breather that is connected to the air filter by a hose. Any
blow-by gases that escape from the breather when this system is used will be aspirated
into the carburetor and reburned.