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TM 9-8000
crankcase of blowby fumes.  The fumes are then
valve body.  This will seal the crankcase from the
aspirated back into the engine where they are reburned.
backfire, which could otherwise cause an explosion.
(1) A hose is tapped into the crankcase at a
point that is well above the engine oil level. The other
(5) The positive crankcase ventilation system
end of the hose is tapped into the intake manifold or the
can be the open or the closed type.
base of the carburetor. It should be noted that if the hose
is tapped into the carburetor base, it will be in a location
(a) The open type has an inlet breather that
that is between the throttle valves and the intake manifold
is open to the atmosphere. When this system is used, it
so that it will receive manifold vacuum.
is possible for a portion of the crankcase blowby to
escape through the breather whenever the engine is
An inlet breather is installed on the
under a sustained heavy load. This is unacceptable on
crankcase in a location that is well above the level of the
later automotive equipment and, as a result, the system
engine oil. The inlet breather also is located strategically
is no longer used.
to ensure complete purging of the crankcase by fresh air.
(b) The closed type has a sealed breather
(3)  The areas of the crankcase where the
that is connected to the air filter by a hose. Any blowby
vacuum hose and the inlet breather are tapped have
gases that escape from the breather when this system
baffles to keep the motor oil from leaving the crankcase.
is used will be aspirated into the carburetor and burned.
This Is the system that is currently used.
(4) A flow control valve is installed in the line
that connects the crankcase to the manifold vacuum. It
7-8. Catalytic Converters (Fig. 7-8).
is called a positive crankcase ventilation (PCV) valve and
serves to avoid the air-fuel mixture by doing the following:
a.  Purpose.  As stated in paragraph 7-5, it is
virtually impossible to keep carbon monoxide and
(a) Any period of large throttle opening will
hydrocarbon  emissions  at  acceptable  levels  by
be accompanied by heavy engine loads.  Crankcase
controlling them in the cylinder without shortening engine
blowby will be at its maximum during heavy engine loads.
life considerably.  It has been found that the most
The PCV valve will react to the small amount of manifold
practical method of controlling these emissions is outside
vacuum that also is present during heavy engine loading
of the engine in a device called a catalytic converter. The
by opening fully through the force of its control valve
catalytic converter is a device that is similar in
spring.  In this way, the system provides maximum
appearance to a muffler. It is positioned in the exhaust
effectiveness during maximum blowby periods.
system, usually between the engine and the muffler. As
the engine exhaust passes through the converter, carbon
(b) Any period of small throttle opening will
monoxide and hydrocarbons are oxidized (combined with
be accompanied by small engine loads, high manifold
oxygen), changing them to carbon dioxide and water.
vacuum, and a minimum amount of crankcase blowby.
During these periods, the high manifold vacuum will pull
b.  Construction and Operation.  The catalytic
the PCV valve to its position of minimum opening. This
converter contains a material (usually platinum or
is Important to prevent an excessively lean air-fuel
palladium) that acts as a catalyst.
A catalyst is
something  that  causes  a  reaction  between  two
substances without actually getting involved. In the case
(c) In the event of engine backfire (flame
of the catalytic converter, oxygen is joined chemically
traveling  back  through  the  intake manifold), the
with carbon monoxide and hydrocarbons in the presence
reverse pressure will push the rear shoulder of the
of its catalyst.
control valve against the
(1) The oxidation process that occurs within
the catalytic converter generates a


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